M2a Cells Research Articles

Endoplasmic reticulum stress drives macrophages to produce IL-33 to favor Th2 polarization in the airways.

Interleukin (IL)-33 is a key driver of T helper 2 (Th2) cell polarization. Endoplasmic reticulum (ER) stress plays a role in the skewed T cell activation. The objective of this project is to elucidate the role of IL-33 derived from macrophages in inducing Th2 polarization in the airways. In this study, bronchoalveolar lavage fluids (BALF) were collected from patients with asthma and healthy control subjects. Macrophages were isolated from the BALF by flow cytometry cell sorting. An asthmatic mouse model was established using the ovalbumin/alum protocol. The results showed that increased IL33 gene activity and ER stress-related molecules in BALF-derived M2a macrophages was observed in asthmatic patients. Levels of IL33 gene activity in M2a cells were positively correlated with levels of asthma response in asthma patients. Sensitization exacerbated the ER stress in the airway macrophages, which increased the expression of IL-33 in macrophages of airway in sensitized mice. Conditional ablation of Il33 or Perk or Atf4 genes in macrophages prevented induction of airway allergy in mice. In conclusion, asthma airway macrophages express high levels of IL-33 and at high ER stress status. Inhibition of IL-33 or ER stress in macrophages can effectively alleviate experimental asthma.

Effect of Differently Polarized Human Macrophages on the SH-SY5Y Cells Damaged by Ischemia and Hypoxia In Vitro.

Macrophages are the major cells of innate immunity with a wide range of biological effects due to their great plasticity and heterogeneity. Macrophages play a key role in neuroregeneration following nervous tissue injury. However, the neuroregenerative potential of various macrophage phenotypes, including those polarized by efferocytosis, remains unexplored. The aim of this study was to compare the neuroregenerative and neuroprotective activity of soluble factors secreted by variously activated human macrophages on the functions of neural progenitors in an in vitro model of ischemia or ischemia/hypoxia. Macrophages were polarized by interferon-γ (M1), IL-4 (M2a), or interaction with apoptotic cells (M2(LS)). The effect of macrophages conditioned media on the proliferation, differentiation, and survival of SH-SY5Y cells damaged by serum deprivation alone (ischemic conditions) or in combination with CoCl2 (ischemic/hypoxic conditions) was assessed. All studied macrophages stimulated the proliferation and differentiation of SH-SY5Y cells. On day 3, the pro-proliferating effect of M1 and M2 was similar and did not depend on the severity of the damaging effect (ischemia or ischemia/hypoxia), while on day 7 and under ischemic/hypoxic conditions, the effects of M2(LS) exceeded those of M1 and M2a cells. The prodifferentiation effects of macrophages were manifested in both short- and long-term cultures, mainly under ischemic/hypoxic conditions, and were most characteristic of M2(LS) cells. Importantly, the ischemia/hypoxia model was accompanied by the pronounced death of SH-SY5Y cells. Only macrophages with the M2 phenotype demonstrated antiapoptotic activity, and the effect of M2(LS) was higher than that of M2a. The results obtained indicate that human macrophages have neuroprotective and neuroregenerative activity, which is mediated by soluble factors, is most characteristic for macrophages activated by efferocytosis (M2(LS)), and is most prominent under in vitro conditions simulating the combined effect of ischemia/hypoxia.

Arsenite inhibits M2a polarization of macrophages through downregulation of peroxisome proliferator-activated receptor gamma

Arsenite (As+3) is a group one human carcinogen, which has been associated with many diseases. Previous studies indicated that As+3 could inhibit wound healing and repair. M2a cells are known as tissue remodeling macrophages, which play an important role in wound repair process. Peroxisome proliferator-activated receptor gamma (PPAR-γ), a key regulator of lipid and glucose metabolism, was found to mediate the IL-4-dependent M2a polarization of macrophages. In the present study, As+3 induced dose-dependent inhibition of M2a polarization starting from 0.1 μM in THP-1-derived macrophages stimulated with 20 ng/mL IL-4. Increased lipid accumulation and decreased PPAR-γ expression were also observed in As+3-treated M2a macrophages. Rosiglitazone (RSG), a potent PPAR-γ agonist, alleviated the suppressions of PPAR-γ and M2a polarization induced by 2 μM As+3. Collectively, these results not only demonstrated that As+3 was able to inhibit polarization of M2a cells through PPAR-γ suppression, but also indicated that PPAR-γ could be utilized as a target for the prevention and treatment of As+3-induced immunotoxicity.

The Secretome of Preconditioned Mesenchymal Stem Cells Drives Polarization and Reprogramming of M2a Macrophages toward an IL-10-Producing Phenotype.

The preconditioning of mesenchymal stem cells (MSCs) has been recognized as an attractive tool to improve their regenerative and immunomodulatory capacities based on their paracrine effects. In this study, we examined the potential of an MSC-conditioned medium (MSC-CM) to alter the phenotype of murine macrophages and to drive reprogramming toward an anti-inflammatory, M2-like state in vitro. We further explored the impact of MSC cytokine preconditioning on the immunosuppressive properties of the MSC secretome. The MSC-CM suppressed the expression of proinflammatory genes in murine M1 macrophages, but only the CM from preconditioned MSCs (preMSC-CM) downregulated their expression during M1 polarization. Remarkably, only the preMSC-CM significantly increased the expression of M2a-, M2b- and M2c-specific genes and proteins during M2a polarization. Further, macrophages were found to secrete high levels of anti-inflammatory IL-10. Similarly, M2a macrophages cultured in the presence of the preMSC-CM displayed an enhanced expression of M2b/M2c-specific markers, suggesting that the secretome of preMSC promotes the repolarization of M2a-like macrophages to M2b/M2c subtypes. The preMSC-CM was found to be enriched in molecules involved in M2 polarization. Additionally, a unique downregulation of extracellular matrix components was observed. Altogether, the preMSC-CM may provide an attractive strategy to dampen inflammation by suppressing the expression of proinflammatory mediators and promoting the polarization and phenotype switch of M2a cells to IL-10-secreting M2b/M2c-like macrophages.

IgG4 drives M2a macrophages to a regulatory M2b-like phenotype: potential implication in immune tolerance.

BackgroundMacrophages can be converted in vitro into immunoregulatory M2b macrophages in the presence of immune complexes (ICs), but the role of the specific subclasses IgG1 or IgG4 in this phenotypic and functional change is not known.ObjectiveWe aimed to refine the original method by applying precisely defined ICs of the subclasses IgG4 or IgG1 constructed by two independent methods.MethodsMonocyte‐derived macrophages (MDMs) were treated with M‐CSF, followed by IL‐4/IL‐13 to induce the M2a allergic phenotype. To mimic unspecific or allergen‐specific ICs, plates were coated with myeloma IgG1 or IgG4, or with grass pollen allergen Phl p 5 followed by recombinant human Phl p 5‐specific IgG1 or IgG4. M2a polarized macrophages were then added, cultured, and examined for cellular markers and cytokines by flow cytometry, ELISA, and rtPCR. Alternatively, immune complexes with IgG1 or IgG4 were formed using protein L.ResultsIgG4 ICs down regulated CD163 and CD206 on M2a cells, and significantly increased IL‐10, IL‐6, TNFα, and CCL1 secretion, indicating a shift to an M2b‐like phenotype. Treatment with IgG4 ICs resulted in expression of FcγRII and down modulation of FcγRII compared with IgG1 treated cells (P = 0.0335) or untreated cells (P < 0.00001).ConclusionImmune complexes with subclasses IgG1 and IgG4 can in vitro be generated by plate absorption, and in fluid form by protein L. Cross‐linking of FcγRIIb by the IgG4 subclass redirects pro‐allergic M2a macrophages to an M2b‐like immunosuppressive phenotype. This suggests an interplay of macrophages with IgG4 in immune tolerance, likely relevant in allergen immunotherapy.

Antibody Cross-Linking of CD14 Activates MerTK and Promotes Human Macrophage Clearance of Apoptotic Neutrophils: the Dual Role of CD14 at the Crossroads Between M1 and M2c Polarization.

Mer receptor tyrosine kinase (MerTK) is key for efficient phagocytosis of apoptotic neutrophils (ANs) and homeostasis of IL-10 production by human anti-inflammatory M2c monocytes/macrophages. We asked whether stimulation of M2c surface receptors contributes in turn to MerTK activation. For this purpose, humanmonocytes/macrophages were differentiated under M1, M2a, and M2c polarizing conditions. The effects of antibody-mediated cross-linking of M2c receptors (i.e., CD14, CD16, CD32, CD163, CD204) on MerTK phosphorylation and phagocytosis of ANs were tested. MerTK expression was also studied by flow cytometry and western blot in the presence of LPS and in M2c-derived microvesicles (MVs). Antibody cross-linking of either CD14 or CD32/FcγRII led to Syk activation and MerTK phosphorylation in its two distinct glycoforms (175-205 and 135-155kDa). Cross-linked CD14 enhanced efferocytosis by M2c macrophages and enabled M1 and M2a cells to clear ANs efficiently. In M1 conditions, LPS abolished surface MerTK expression on CD14bright cell subsets, so disrupting the anti-inflammatory pathway. In M2c cells, instead, MerTK was diffusely and brightly co-expressed with CD14, and was also detected in M2c macrophage-derived MVs; in these conditions, LPS only partially downregulated MerTK on cell surfaces, while the smaller MerTK glycoform contained in MVs remained intact. Altogether, cooperationbetween CD14 and MerTK may foster the clearance of ANs by human monocytes/macrophages. CD14 stands between M1-related LPS co-receptor activity and M2c-related MerTK-dependent response. MerTK interaction with CD32/FcγRII, its detection in M2c MVs, and the differential localization and LPS susceptibility of MerTK glycoforms add further new elements to the complexity of the MerTK network.

Canine macrophages can like human macrophages be in vitro activated toward the M2a subtype relevant in allergy

The M2a subtype of macrophages plays an important role in human immunoglobulin E (IgE-mediated allergies) and other Th2 type immune reactions. In contrast, very little is known about these cells in the dog. Here we describe an in vitro method to activate canine histiocytic DH82 cells and primary canine monocyte-derived macrophages (MDMs) toward the M2a macrophages using human cytokines. For a side-by-side comparison, we compared the canine cells to human MDMs, and the human monocytic cell line U937 activated towards M1 and M2a cells on the cellular and molecular level. In analogy to activated human M2a cells, canine M2a, differentiated from both DH82 and MDMs, showed an increase in CD206 surface receptor expression compared to M1. Interestingly, canine M2a, but not M1 derived from MDM, upregulated the high-affinity IgE receptor (FcεRI). Transcription levels of M2a-associated genes (IL10, CCL22, TGFβ, CD163) showed a diverse pattern between the human and dog species, whereas M1 genes (IDO1, CXCL11, IL6, TNF-α) were similarly upregulated in canine and human M1 cells (cell lines and MDMs). We suggest that our novel in vitro method will be suitable in comparative allergology studies focussing on macrophages.

Mesenchymal stem cells promote macrophage polarization toward M2b-like cells

Mesenchymal stem or stromal cells (MSCs) act on different components of the immune response including macrophages (MΦs). Therefore this study has been committed to explore how MSCs may modify the effect of MΦ polarization upon an inductive environment using mouse bone marrow (BM)-derived “naïve”, unpolarized MΦs. Phagocytosis of various MΦ subtypes was different since M1 and M2b showed poorer, while M2a higher rate of phagocytosis. MSCs significantly promoted yeast ingestion by M1 and M2b and diminished it by M2a cells. Under polarizing conditions, MSCs profoundly affected the TNFα production of MΦ subtypes since M1 and M2b MΦs produced less and M2a produced higher amount of TNFα while the amount of IL-10 was not affected. The most striking effect of MSCs was registered on M2b cells since the inflammatory TNFα dominance remarkably shifted to the immunosuppressive IL-10. Prepolarized M1 cells readily converted to M2a and M2b states when polarizing conditions changed from M1 to M2a or M2b induction, respectively. Repolarizing from M1 to M2a resulted in the decline of IL-10 and TNFα and defined elevation of Ym1 similar to levels characteristic to M2a primarily polarized from naïve BM-MΦs. Similarly, polarization of M1 to M2b MΦs was successful showing increase in IL-10 and reduction in TNFα levels characteristic to M2b cells. However, when co-culturing with MSCs, M1-M2a or M1-M2b transition was not affected. Crosstalk between MΦs and MSCs depended on PGE-2 since COX-2 inhibition reduced the effect of MSCs to establish an IL-10-dominant cytokine production by MΦs.

Update on the role of alternatively activated macrophages in asthma.

Lung macrophages link innate and adaptive immune responses during allergic airway inflammatory responses. Alveolar macrophages (AMs) and interstitial macrophages are two different phenotypes that differentially exert immunological function under physiological and pathological conditions. Exposure to pathogen induces polarization of AM cells into classically activated macrophages (M1 cells) and alternatively activated macrophages (M2 cells). M1 cells dominantly express proinflammatory cytokines such as TNF-α and IL-1 β and induce lung inflammation and tissue damage. M2 cells are further divided into M2a and M2c subsets. M2a cells dominantly produce allergic cytokines IL-4 and IL-13, but M2c cells dominantly produce anti-inflammatory cytokine IL-10. M2a and M2c cells are differently involved in initiation, inflammation resolution, and tissue remodeling in the different stages of asthma. Microenvironment dynamically influences polarization of AM cells. Cytokines, chemokines, and immune-regulatory cells interplay and affect the balance between the polarization of M1 and M2 cells, subsequently influencing disease progression. Thus, modulation of AM phenotypes through molecular intervention has therapeutic potential in the treatment of asthma and other allergic inflammatory diseases. This review updated recent advances in polarization and functional specialization of these macrophage subtypes with emphasis on modulation of polarization of M2 cells in asthma of human subjects and animal models.

TGF-β1 produced by alternatively activated macrophages synchronizes a tolerogenic iTreg-Th17 cell axis

Abstract Induced regulatory T (iTreg) cells and T helper 17 (Th17) cells develop at mucosal interfaces. While iTreg cells are required for the maintenance of mucosal tolerance, Th17 cells can play divergent roles. Both iTreg and Th17 cells require TGF-β1 for their differentiation. Importantly, the primary source of TGF-β1 in vivo is unknown. Here, we investigate the mechanisms that control iTreg and Th17 cell development. We hypothesize that alternatively activated M2a macrophages support mucosal tolerance through TGF-β1 mediated expansion of both iTreg and Th17 cells. We pretreat Rag1­­−/− C57BL/6 mice prior to colitis induction or treat mice with established disease with polarized macrophages. In some experiments the macrophages lacked the capacity to produce TGF-β1. Weight loss, colitis disease scores, and FACS analysis of mesenteric lymph nodes and gut infiltrates were used to evaluate the impact of macrophage therapy on the expansion and function of the iTreg-Th17 cell axis. Pre-conditioning mice with M2a but not M1 macrophages resulted in a 16-fold increase in the number of iTreg cells and a 8-fold increase in the number of Th17 cells when compared to control mice. This expansion required TGF-β1 produced by both the transferred M2a cells and by host myeloid cells. Established colitis was successfully treated by concomitant M2a and nTreg cell transfers, but not by nTreg cell transfers alone. In conclusion, M2a macrophages that produce TGF-β1 regulate development of a tolerogenic iTreg-Th17 cell axis. The change in iTreg and Th17 cells numbers linked to M2a macrophage polarization is directionally concordant and dependent on T cell produced IL17A, confirming that both iTreg and Th17 cells are required to mediate intestinal homeostasis.

Dendritic cell-specific intercellular adhesion molecule-3 grabbing nonintegrin mediates HIV-1 infection of and transmission by M2a-polarized macrophages in vitro

To assess in-vitro effects of monocyte-derived macrophage (MDM) polarization into M1 and M2a cells on HIV-1 replication and transmission and obtain new insights into the potential importance of macrophage polarization in vivo. Human peripheral blood monocytes were differentiated into MDM for 7 days. Control and MDM polarized into M1 or M2a cells were exposed to different strains of HIV-1 and assessed for their ability to bind and transmit virus to CD4 T lymphocytes. MDM were incubated with either tumour necrosis factor-alpha (TNF-α) along with interferon-gamma (IFN-γ) or with interleukin-4 (IL-4) for 18 h to obtain M1 or M2a cells, respectively. Expression of cell surface antigens, including CD4 and dendritic cell-specific intercellular adhesion molecule-3 grabbing nonintegrin (DC-SIGN), was evaluated by flow cytometry. C-C chemokine receptor type 5 (CCR5)-dependent (R5) HIV-1 binding, DNA synthesis and viral replication were assessed in the presence or absence of anti-DC-SIGN blocking mAbs. Transmission of C-X-C chemokine receptor type 4 (CXCR4)-dependent (X4) and R5 HIV-1 from MDM to IL-2 activated CD4 T cells was also investigated. DC-SIGN was strongly upregulated on M2a-MDM and downregulated on M1-MDM compared with control MDM. DC-SIGN facilitated HIV-1 entry and DNA synthesis in M2a-MDM, compensating for their low levels of CD4 cell expression. M2a-MDM efficiently transmitted both R5 and X4 HIV-1 to CD4 T cells in a DC-SIGN-dependent manner. DC-SIGN facilitates HIV-1 infection of M2a-MDM, and HIV-1 transfer from M2a-MDM to CD4 T cells. M2a-polarized tissue macrophages may play an important role in the capture and spread of HIV-1 in mucosal tissues and placenta.

Mo2070 Modulatory Effect of Hypoxia on Inflammatory and Angiogenic Activation of Human Intestinal Microvascular Endothelial Cells

M2a was four fold increased. Interleukin-6 transcript level in inflammatory M1 cells was reduced two fold after polarization with IFN-γ and TSA. In M2a cells expressed significantly lower HDAC 7 levels compared to M1 cells (p<0.05) after 3 days of polarization. Human M1, M2a and M2c macrophages expressed the polarization markers CD64, CD200R and CD163 respectively. No significant differences were found in HDAC 1-11 and sirtuin 1 expression levels between MDM subsets directly after polarization. LPS stimulation of MDM in the presence of TSA showed that HDAC inhibition had no effect on TNF-α or IL-6 secretion, but IL-10 secretion was abrogated almost completely in all polarized macrophage MDM subsets. CONCLUSIONHDAC activity is involved inmacrophage polarization process. Inhibition of HDAC activity changes cytokine responses of macrophage M2a to inflammatory stimuli. Together, these data underscore the potential of HDAC inhibition to intervene in macrophage differentiation and reactivity in inflammatory disease.

Mo2069 Effect of Intestinal Extracelluar Matrix on Endothelial Biology and Angiogenesis

M2a was four fold increased. Interleukin-6 transcript level in inflammatory M1 cells was reduced two fold after polarization with IFN-γ and TSA. In M2a cells expressed significantly lower HDAC 7 levels compared to M1 cells (p<0.05) after 3 days of polarization. Human M1, M2a and M2c macrophages expressed the polarization markers CD64, CD200R and CD163 respectively. No significant differences were found in HDAC 1-11 and sirtuin 1 expression levels between MDM subsets directly after polarization. LPS stimulation of MDM in the presence of TSA showed that HDAC inhibition had no effect on TNF-α or IL-6 secretion, but IL-10 secretion was abrogated almost completely in all polarized macrophage MDM subsets. CONCLUSIONHDAC activity is involved inmacrophage polarization process. Inhibition of HDAC activity changes cytokine responses of macrophage M2a to inflammatory stimuli. Together, these data underscore the potential of HDAC inhibition to intervene in macrophage differentiation and reactivity in inflammatory disease.

Defective Macrophage Function in Crohn's Disease: Role of Alternatively Activated Macrophages in Inflammation

Introduction The aetiology of Crohn9s disease (CD) involves a genetically determined dysregulated immune response to commensal intestinal microflora. In CD, viable E coli are found within lamina propria macrophages (MΦs) and E coli intracellular survival is prolonged in CD-derived MΦs in vitro. Different MΦ subpopulations exist, M1 cells are inflammatory cells, M2a cells are involved in tissue repair and M2c are regulatory cells that limit inflammation. The role these different MΦs play in this abnormal handling of bacteria in CD is unclear. Methods The aim of this study was to examine in vitro M1 and M2 MΦ maturation in CD patients and healthy individuals and how these cells respond to challenge with CD-derived E coli. To do this we monitored MΦ morphology, surface markers and cytokine production and intracellular bacterial survival. Peripheral blood monocytes were isolated from CD patients and healthy controls and treated with cytokines to generate distinct MΦ subpopulations: IFNγ for M1, IL4/IL13 for M2a and IL10 for M2c. MΦ morphology was assessed by H&E staining and surface marker expression of CD14, CD16 and CD33, chemokine receptor CCR2, scavenger receptor CD163, co stimulatory molecule CD40 and mannose receptor CD206 using flow cytometry. IL10 and TNFα production were measured by ELISA and intracellular E coli survival was measured using the gentamicin protection assay. Results In contrast to M1 MΦ9s, both CD-derived M2 populations failed to develop the characteristic MΦ dendrite morphology seen in control macrophages. Surface expression of CD40 in M2 CD-derived MΦs was 3.4-fold lower for M2a and 4.4-fold lower for M2c compared to controls after E coli challenge. CD163 was higher in M1 CD cells but reduced by 50% in M2 cells compared to healthy cells. After E coli challenge, TNFα production by M2 but not M1 MΦs was significantly lower in CD than in controls (M2a 38%, M2c 27% respectively less than controls) but there were no differences in IL10 production. Prolonged intracellular survival of E coli was demonstrated in CD M2 cells but effective killing occurred in all M1 CD MΦs and all control MΦs. Conclusion In CD, M2 (but not M1) MΦs display abnormal morphological maturation, lower TNFα levels after E coli challenge, prolonged intracellular bacterial survival and differences in surface marker expression. The results are consistent with an innate MΦ defect in CD relating particularly to a failure of the normal role of M2 MΦs to limit and control inflammation.

Defective macrophage function in crohn's disease: role of alternatively activated macrophages in inflammation

IntroductionThe aetiology of Crohn's disease (CD) involves a genetically determined dysregulated immune response to commensal intestinal microflora. In CD, viable E coli are found within lamina propria macrophages (MΦs) and...

W1799 Defective In Vitro Macrophage Maturation in Crohn's Disease

Introduction Whist the cause of Crohn9s disease (CD) remains unclear, an innate defect in the handling of intracellular bacteria appears to be involved. Evidence for this comes from observations of a defect in phagocyte recruitment and the presence of viable E coli within lamina propria (LP) macrophages in these patients. LP macrophages are derived from blood monocytes and develop into pro inflammatory cells under the influence of Th1 cytokine (M1) or anti inflammatory regulatory macrophages (M2) by Th2 cytokines. It is known that macrophage turnover in CD is higher and maturation incomplete compared to those in healthy individuals so these cells are a good candidate for the observed differences. Methods To study in vitro the cytokine-driven differentiation of macrophages from patients with IBD (5 CD, 5 UC) and five healthy controls and follow their behaviour when challenged with CD derived strains of E coli . Monocytes were stimulated for 6 days with INF-γ, IL-4 + IL-13 or IL-10 to generate M1, M2a or M2c macrophage populations, respectively. Specific maturation markers (CD14, CD16, CD33, CD163, CD206, CD209, CCR2 and CCR7) were then measured using flow cytometry. These cells were then challenged with E coli and phagocytic function measured using the gentamicin protection assay. Results Monocytes stimulated with INFγ, from both the control and IBD groups gave a similar M1 type profile. IL-4+IL-13 stimulation gave rise to M2a cells, however the cells isolated from the CD group expressed a higher percentage of CD14 and a greater proportion of these cells expressed low levels of CD163 and CD206. In the CD group, IL-10 gave rise to a population that expressed CD14+, CD16+ and were CD163++, CD206+ and CD209+. When these different cell populations were challenged with E coli the number of recovered viable bacteria at 30 min gave similar numbers of internalised bacteria. However when we recovered bacteria 3 h post infection, significantly higher survival of bacteria in the IL-10, M2c population in the CD Group (M1 cells, 14±9; M2a cells, 23±8; M2c cells, 135±41) was found. No bacteria survived in the controls or UC. Conclusion We have demonstrated a population of IL-10 induced M2c cells that are unable to complete the process of bacterial killing effectively in patients with CD. These CD163+ cells are similar to a subpopulation of lamina propria macrophages that have been shown harbour viable bacteria. This may indicate that a defect in the migration and maturation of monocytes, unable to clear bacteria efficiently, may be the underlying innate defect in CD.

Macrophage polarization and HIV-1 infection

Polarization of MP into classically activated (M1) and alternatively activated (M2a, M2b, and M2c) macrophages is critical in mediating an effective immune response against invading pathogens. However, several pathogens use these activation pathways to facilitate dissemination and pathogenesis. Viruses generally induce an M1-like phenotype during the acute phase of infection. In addition to promoting the development of Th1 responses and IFN production, M1 macrophages often produce cytokines that drive viral replication and tissue damage. As shown for HIV-1, polarization can also alter macrophage susceptibility to infection. In vitro polarization into M1 cells prevents HIV-1 infection, and M2a polarization inhibits viral replication at a post-integration level. M2a cells also express high levels of C-type lectins that can facilitate macrophage-mediated transmission of HIV-1 to CD4(+) T cells. Macrophages are particularly abundant in mucosal membranes and unlike DCs, do not usually migrate to distal tissues. As a result, macrophages are likely to contribute to HIV-1 pathogenesis in mucosal rather than lymphatic tissues. In vivo polarization of MP is likely to span a spectrum of activation phenotypes that may change the permissivity to and alter the outcome of HIV-1 and other viral infections.

Shifts in macrophage phenotypes and macrophage competition for arginine metabolism affect the severity of muscle pathology in muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common, lethal, muscle-wasting disease of childhood. Previous investigations have shown that muscle macrophages may play an important role in promoting the pathology in the mdx mouse model of DMD. In the present study, we investigate the mechanism through which macrophages promote mdx dystrophy and assess whether the phenotype of the macrophages changes between the stage of peak muscle necrosis (4 weeks of age) and muscle regeneration (12 weeks). We find that 4-week-old mdx muscles contain a population of pro-inflammatory, classically activated M1 macrophages that lyse muscle in vitro by NO-mediated mechanisms. Genetic ablation of the iNOS gene in mdx mice also significantly reduces muscle membrane lysis in 4-week-old mdx mice in vivo. However, 4-week mdx muscles also contain a population of alternatively activated, M2a macrophages that express arginase. In vitro assays show that M2a macrophages reduce lysis of muscle cells by M1 macrophages through the competition of arginase in M2a cells with iNOS in M1 cells for their common, enzymatic substrate, arginine. During the transition from the acute peak of mdx pathology to the regenerative stage, expression of IL-4 and IL-10 increases, either of which can deactivate the M1 phenotype and promote activation of a CD163+, M2c phenotype that can increase tissue repair. Our findings further show that IL-10 stimulation of macrophages activates their ability to promote satellite cell proliferation. Deactivation of the M1 phenotype is also associated with a reduced expression of iNOS, IL-6, MCP-1 and IP-10. Thus, these results show that distinct subpopulations of macrophages can promote muscle injury or repair in muscular dystrophy, and that therapeutic interventions that affect the balance between M1 and M2 macrophage populations may influence the course of muscular dystrophy.