Distinct Macrophage Populations Research Articles

Investigating the Roles of Donor Macrophages after Cardiac Transplantation

Heart transplantation is limited by donor availability, immune mediated rejection, and unintended adverse effects from immunosuppression. Expanding donor availability and immunologic compatibility are priorities. Donor-organ directed therapies have the potential to improve allograft survival while minimizing patient harm. The heart contains 2 ontogenically distinct tissue resident macrophage populations (CCR2- and CCR2+ macrophages) with divergent inflammatory and reparative functions. The exact role of donor macrophages after cardiac transplant remains unexplored. A better understanding of how donor intrinsic immunity influences allograft acceptance and survival may provide new opportunities to improve the outcomes of transplant recipients. We utilized a fully HLA-mismatched mouse heart transplant model in combination with two differing immunosuppression regimens: CTLA4-Ig (acute rejection model) and CTLA4 Ig + anti-CD40L Ab (graft tolerance model). We assessed donor macrophage composition, location, and dynamics at various time points following transplantation using genetic lineage tracing and imaging. The functional requirements for CCR2- and CCR2+ donor macrophages was assessed using transgenic mice that facilitate selective depletion of CCR2- or CCR2+ macrophages. We show that donor CCR2- and CCR2+ macrophages persist in the heart following transplantation. We demonstrate that donor CCR2- and CCR2+ macrophages display dynamic responses following IRI and rejection. Co-labelling with CD68 highlights the later composition of donor-derived CCR2+ macrophages, donor-derived CCR2- macrophages, and resident monocyte derived macrophages in the transplanted heart. Depletion of CCR2+ macrophages prior to transplantation resulted in prolonged allograft survival and reduced cardiomyocyte cell death. In contrast, deletion of CCR2- macrophages led to accelerated rejection, increased cardiomyocyte cell death, and exaggerated leukocyte infiltration. Donor-derived CCR2+ macrophages and donor-derived CCR2- macrophages persist in the cardiac transplant. Depletion of donor-derived CCR2+ macrophages led to graft rejection whereas removal of donor-derived CCR2- macrophages promoted graft survival. These data indicate that donor macrophages play key roles in acute cellular rejection after cardiac transplantation.

Abstract A92: Analysis of lung-resident macrophages as potential regulators of disseminated tumor cell fate

Abstract The majority of cancer-related deaths are due to metastasis at secondary organs originating from solitary disseminated tumor cells (DTCs), which can remain dormant for several years. In the lung, some DTCs undergo a period of dormancy before proliferation to form metastasis, while others will not grow into metastatic lesions. As macrophages are key immune components of the microenvironment at distant sites, we aim to understand their potential role in regulating DTC dormancy and awakening. To test the contribution of lung macrophages to cancer cell proliferation, we first compared the effect of lung-resident alveolar macrophages (AMs) and bone marrow-derived monocytes (BM MOs) on cancer cell phenotypes. Our results indicate that AMs may suppress growth and induce EMT (CDH1lo/TWIST1hi phenotype) in cancer cells (of both early and late stages), while BM MOs promote growth. We further assessed the effect of these distinct macrophage populations on survival and proliferation of single cancer cells in a 3D matrigel assay to mimic single DTCs in the lung. Here, AMs appear to retain cancer cells as single cells, while BM MOs favor a switch to proliferation and colony growth. We further used conditioned media (CM) from lung cultures from naive and macrophage-depleted animals to study how organ-specific cues from macrophages affected tumor cell growth. We performed a proteomic analysis of the lung secretome and identified DKK3 as differentially produced before and after macrophage depletion. DKK3, a regulator of Wnt signaling, was absent in the depleted condition. We then tested its effect on cell proliferation in mammary cancer cell sphere formation assays, and we observed that DKK3 increased sphere formation and growth of early- and late-stage cells, respectively. We will next test whether DKK3 production is derived from AMs, BM MOs, or other lung resident cells and we will profile AMs and BM MOs from early and late stages of tumor progression to determine their gene expression profiles at different times of DTC colonization. To explore the role of AMs in dormancy in vivo, we will perform AM depletion in the CD169-DTR animal model. Here, we will deplete specifically tissue-resident AMs and determine DTC fate after tail-vein injection of breast cancer cells. Our study will thus allow us to determine the contribution of different macrophage populations to metastatic dormancy. Citation Format: Erica Dalla, Maria Casanova-Acebes, Margaret Hung, Nina Linde, Miriam Merad, Julio Aguirre-Ghiso. Analysis of lung-resident macrophages as potential regulators of disseminated tumor cell fate [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr A92.

The omentum, a niche for premetastatic ovarian cancer.

The work by Etzerodt et al. in this issue of JEM (https://doi.org/10.1084/jem.20191869) identifies a distinct omentum-resident macrophage population of embryonic origin and demonstrates that these cells provide a niche for ovarian cancer metastasis and cancer stemness. This research opens up for many questions and therapeutic prospects.

Loss of Rnf43 to accelerate KRAS-mediated neoplasia in a clinically relevant genetically engineered mouse model of pancreatic adenocarcinoma.

733 Background: The genetic heterogeneity of pancreatic ductal adenocarcinoma (PDAC) demands a personalized molecular-targeted treatment approach. While activating KRAS mutations are a near ubiquitous event in PDAC pathogenesis, 5-10% of cases display deleterious driver mutations in the Wnt-signaling negative regulator, ring finger 43 (RNF43). Despite this characterization there are no personalized treatment options for this subset of patients. Methods: We have developed a genetically engineered mouse model (GEMM) of PDAC, driven by an activating mutation in Kras and deletion of Rnf43 under control of a pancreas specific promoter (KRC). Mice were followed for survival outcomes and histological changes which were compared to a Kras driven PDAC GEMM (KC). Mice underwent serial magnetic resonance imaging (MRI), with and without dynamic contrast enhanced (DCE) imaging, to evaluate cystic tumor morphology and contrast enhancement during tumor progression. Single cell RNA sequencing (scRNAseq) was also performed to assess changes in single cell populations during tumor progression. Lastly, we established ex vivo cultures from KRC and KC tumors and performed bulk RNA-sequencing (RNAseq) and in vitro pharmacology studies. Results: KRC mice displayed a decrease in overall survival and higher incidence of both high grade pre-neoplastic lesions and invasive PDAC compared to KC mice. Serial MRI revealed increased cystic morphology of KRC mice during tumor progression with increasing DCE intensity. scRNAseq of KRC tumors from moribund mice displayed two distinct populations of both macrophages and fibroblasts, similar to our previous report of Kras/Tp53 and Kras/Ink4a driven GEMMs. Lastly, primary cultures from KRC tumors demonstrated increased expression of Wnt-related genes by RNAseq and increased sensitivity to small molecule porcupine inhibition relative to KC cell lines, demonstrating functional Wnt dependence of the KRC system. Conclusions: Rnf43 is a bona fide tumor suppressor gene in PDAC. This GEMM is a novel platform for drug discovery in RNF43-mutated PDAC with the eventual goal of implementing a precision oncology approach in these patients.

Role of donor macrophages after heart and lung transplantation.

Since the 1960s, heart and lung transplantation has remained the optimal therapy for patients with end-stage disease, extending and improving quality of life for thousands of individuals annually. Expanding donor organ availability and immunologic compatibility is a priority to help meet the clinical demand for organ transplant. While effective, current immunosuppression is imperfect as it lacks specificity and imposes unintended adverse effects such as opportunistic infections and malignancy that limit the health and longevity of transplant recipients. In this review, we focus on donor macrophages as a new target to achieve allograft tolerance. Donor organ-directed therapies have the potential to improve allograft survival while minimizing patient harm related to global suppression of recipient immune responses. Topics highlighted include the role of ontogenically distinct donor macrophage populations in ischemia-reperfusion injury and rejection, including their interaction with allograft-infiltrating recipient immune cells and potential therapeutic approaches. Ultimately, a better understanding of how donor intrinsic immunity influences allograft acceptance and survival will provide new opportunities to improve the outcomes of transplant recipients.

Distinct macrophage populations and phenotypes associated with IL-4 mediated immunomodulation at the host implant interface.

The host macrophage response to implants has shown to be affected by tissue location and physio-pathological conditions of the patient. Success in immunomodulatory strategies is thus predicated on the proper understanding of the macrophage populations participating on each one of these contexts. The present study uses an in vivo implantation model to analyze how immunomodulation via an IL-4 eluting implant affects distinct macrophage populations at the tissue-implant interface and how this may affect downstream regenerative processes. Populations identified as F4/80+, CD68+ and CD11b+ macrophages at the peri-implant space showed distinct susceptibility to polarize towards an M2-like phenotype under the effects of delivered IL-4. Also, the presence of the coating resulted in a significant reduction in F4/80+ macrophages, while other populations remained unchanged. These results suggests that the F4/80+ macrophage population may be predominant in the early stages of the host response at the surface of these implants, in contrast to CD11b+ macrophage populations which were either fewer in number or located more distant from the implant surface. Gene expression assays showed increased proteolytic activity and diminished matrix deposition as possible mechanisms explaining the decreased fibrotic capsule deposition and improved peri-implant tissue quality shown in previous studies using IL-4 eluting coatings. The pattern of M2-like gene expression promoted by IL-4 was correlated with glycosaminoglycan production within the site of implantation at early stages of the host response, suggesting a significant role in this response. These findings demonstrate that immunomodulatory strategies can be utilized to design and implement targeted delivery for improving biomaterial performance.

Macrophages are Metabolically Heterogeneous within the Tumor Microenvironment

Macrophages are often prominently present in the tumor microenvironment, where distinct macrophage populations can differentially affect tumor progression. Although the metabolism of macrophages influences their function, little is known about the metabolic characteristics of tumor-associated macrophage (TAM) subsets. Using transcriptomic and metabolomic analyses, we now reveal that pro-inflammatory MHC-IIhi TAMs display a hampered TCA cycle, while reparative MHC-IIlo TAMs show a higher oxidative and glycolytic metabolism. Both TAM populations preferred lactate over glucose as a carbon source to fuel the TCA cycle. However, while lactate supported the oxidative metabolism in MHC-IIlo TAMs, it decreased the metabolic activity of MHC-IIhi TAMs. Lactate subtly affected the transcriptome of MHC-IIlo TAMs, increased L-arginine metabolism and enhanced T-cell suppressive capacity of these TAMs. Overall, our data uncover the metabolic intricacies of distinct TAM subsets and identify lactate as an important carbon source, and metabolic and functional regulator of TAMs.

Regulatory Macrophages Inhibit Alternative Macrophage Activation and Attenuate Pathology Associated with Fibrosis.

Diversity and plasticity are the hallmarks of macrophages. The two most well-defined macrophage subsets are the classically activated macrophages (CAMϕs) and the IL-4-derived alternatively activated macrophages (AAMϕs). Through a series of studies, we previously identified and characterized a distinct population of macrophages with immunoregulatory functions, collectively termed regulatory macrophages (RMϕs). Although considerable advances have been made in understanding these various macrophage subsets, it is not known whether macrophages of one activation state can influence the other. In this study, we examined whether RMϕs capable of inhibiting inflammatory responses of CAMϕs could also inhibit AAMϕs and their profibrotic responses. Our results demonstrated that RMϕs significantly dampened the alternate activation phenotype of AAMϕs generated in vitro and intrinsically occurring AAMϕs from TACI-/- macrophages. Further, RMϕs inhibited AAMϕ-promoted arginase activity and fibroblast proliferation in vitro. This inhibition occurred regardless of the strength, duration, and mode of alternative activation and was only partially dependent on IL-10. In the chlorhexidine gluconate-induced peritoneal fibrosis model, AAMϕs worsened the fibrosis, but RMϕs rescued mice from AAMϕ-mediated pathological conditions. Taken together, our study demonstrates that RMϕs are a specialized subset of macrophages with a nonredundant role in limiting overt proregenerative functions of AAMϕs, a role distinct from their well-defined role of suppression of inflammatory responses by CAMϕs.

A spatially restricted fibrotic niche in pulmonary fibrosis is sustained by M-CSF/M-CSFR signalling in monocyte-derived alveolar macrophages.

Ontologically distinct populations of macrophages differentially contribute to organ fibrosis through unknown mechanisms.We applied lineage tracing, single-cell RNA sequencing and single-molecule fluorescence in situ hybridisation to a spatially restricted model of asbestos-induced pulmonary fibrosis.We demonstrate that tissue-resident alveolar macrophages, tissue-resident peribronchial and perivascular interstitial macrophages, and monocyte-derived alveolar macrophages are present in the fibrotic niche. Deletion of monocyte-derived alveolar macrophages but not tissue-resident alveolar macrophages ameliorated asbestos-induced lung fibrosis. Monocyte-derived alveolar macrophages were specifically localised to fibrotic regions in the proximity of fibroblasts where they expressed molecules known to drive fibroblast proliferation, including platelet-derived growth factor subunit A. Using single-cell RNA sequencing and spatial transcriptomics in both humans and mice, we identified macrophage colony-stimulating factor receptor (M-CSFR) signalling as one of the novel druggable targets controlling self-maintenance and persistence of these pathogenic monocyte-derived alveolar macrophages. Pharmacological blockade of M-CSFR signalling led to the disappearance of monocyte-derived alveolar macrophages and ameliorated fibrosis.Our findings suggest that inhibition of M-CSFR signalling during fibrosis disrupts an essential fibrotic niche that includes monocyte-derived alveolar macrophages and fibroblasts during asbestos-induced fibrosis.

Non-classical tissue monocytes and two functionally distinct populations of interstitial macrophages populate the mouse lung

Resident tissue macrophages (RTM) can fulfill various tasks during development, homeostasis, inflammation and repair. In the lung, non-alveolar RTM, called interstitial macrophages (IM), importantly contribute to tissue homeostasis but remain little characterized. Here we show, using single-cell RNA-sequencing (scRNA-seq), two phenotypically distinct subpopulations of long-lived monocyte-derived IM, i.e. CD206+ and CD206−IM, as well as a discrete population of extravasating CD64+CD16.2+ monocytes. CD206+ IM are peribronchial self-maintaining RTM that constitutively produce high levels of chemokines and immunosuppressive cytokines. Conversely, CD206−IM preferentially populate the alveolar interstitium and exhibit features of antigen-presenting cells. In addition, our data support that CD64+CD16.2+ monocytes arise from intravascular Ly-6Clo patrolling monocytes that enter the tissue at steady-state to become putative precursors of CD206−IM. This study expands our knowledge about the complexity of lung IM and reveals an ontogenic pathway for one IM subset, an important step for elaborating future macrophage-targeted therapies.

Cellular heterogeneity during mouse pancreatic ductal adenocarcinoma progression at single-cell resolution.

Pancreatic ductal adenocarcinoma (PDA) is a major cause of cancer-related death with limited therapeutic options available. This highlights the need for improved understanding of the biology of PDA progression, a highly complex and dynamic process featuring changes in cancer cells and stromal cells. A comprehensive characterization of PDA cancer cell and stromal cell heterogeneity during disease progression is lacking. In this study, we aimed to profile cell populations and understand their phenotypic changes during PDA progression. To that end, we employed single-cell RNA sequencing technology to agnostically profile cell heterogeneity during different stages of PDA progression in genetically engineered mouse models. Our data indicate that an epithelial-to-mesenchymal transition of cancer cells accompanies tumor progression in addition to distinct populations of macrophages with increasing inflammatory features. We also noted the existence of three distinct molecular subtypes of fibroblasts in the normal mouse pancreas, which ultimately gave rise to two distinct populations of fibroblasts in advanced PDA, supporting recent reports on intratumoral fibroblast heterogeneity. Our data also suggest that cancer cells and fibroblasts may be dynamically regulated by epigenetic mechanisms. This study systematically describes the landscape of cellular heterogeneity during the progression of PDA and has the potential to act as a resource in the development of therapeutic strategies against specific cell populations of the disease.

Intestinal Macrophages in Resolving Inflammation

Macrophages not only regulate intestinal homeostasis by recognizing pathogens to control enteric infections but also employ negative feedback mechanisms to prevent chronic inflammation. Hence, macrophages are intriguing targets for immune-mediated therapies, especially when barrier function in the gut is compromised to trigger aberrant inflammatory responses, most notably during inflammatory bowel diseases. Recently, there has been considerable progress in our understanding of human macrophage biology in different tissues, including the intestines. In this review, we discuss some new findings on the properties of distinct populations of intestinal macrophages, how resolution of inflammation and tissue repair by macrophages could be promoted by type 2 cytokines as well as other therapeutic interventions, and highlight some challenges for translating these findings into the future for this exciting area of immunology research.

Abstract 4565: The functional role of atypical chemokine receptor 1 in immune cell regulation of breast cancer

Abstract Breast cancer (BC) is a heterogeneous disease that leads to varied molecular subtypes and distinct clinical outcomes. Tumor heterogeneity, which is in part influenced by genetic ancestry, contributes to racial disparities in BC. This disparity has persisted for over 30 years, despite improvements in detection, screening, and treatment methods. When investigating race-specific differences in the breast tumor microenvironment (TME), we believe that chemokine receptors, such as atypical chemokine receptor 1 (ACKR1/DARC), play an integral role in regulating chemokine and immune cell migration in circulation and the TME, ultimately influencing tumor progression. ACKR1/DARC specifically plays a role in ancestry-related differences in BC due to an African-specific ancestral allele (“Duffy-null” mutation) that is fixed in Sub-Saharan African populations, and present in 60-70% of African-Americans. Those that harbor the mutation do not express ACKR1/DARC on erythrocytes, ultimately affecting immune cell migration in these populations only. To better understand the effects of this variant in circulation and the TME, we performed immunohistochemistry (IHC) on breast tumors from a cohort of patients with matched blood samples. From our IHC analysis, we quantified levels of ACKR1/DARC, in addition to levels of target pro-inflammatory chemokines, and distinct immune populations of T-cells and macrophages. In addition, individuals were genotyped to determine Duffy-null status, and a correlative Luminex multiplex assay was performed on patient plasma to determine concentrations of pro-inflammatory chemokines in circulation. We determined that those with low expression of ACKR1/DARC in tumors exhibited a unique signature of immune cell infiltrates that would encourage a more aggressive TME. In addition, we observed variable levels of circulating chemokines, where those with the Duffy-null mutation exhibited significantly decreased levels of CCL2. Overall, our analyses support the hypothesis that tumor-specific DARC/ACKR1 plays a vital role in immune cell regulation in women with BC. Citation Format: Brittany D. Jenkins, Rachel Martini, Kevin Gardner, Michele Monteil, Dorrah Deeb, Lisa Newman, Melissa Davis. The functional role of atypical chemokine receptor 1 in immune cell regulation of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4565.

Cx3CR1 Expression Identifies Distinct Macrophage Populations That Contribute Differentially to Inflammation and Repair.

Bone marrow (BM)-derived classical monocytes are critical to wound repair, where they differentiate into macrophages and purge foreign materials and dead cells while also laying the framework for tissue repair and regeneration. A subset of this recruited population persists in the wound and acquires alternative activation states to promote cell proliferation and matrix remodeling. In diabetes, this phenotypic switch is impaired and inflammation persists in an elevated state, contributing to delayed wound healing. Long-term tissue-resident macrophages can also play a key role in the resolution of inflammation to varying degrees across different organs. In this study, we investigated different macrophage subpopulations in nondiabetic and diabetic wounds over time using Cx3CR1eGFP transgenic mice and BM transplants. We show Cx3CR1eGFP-hi macrophages in skin wounds are derived from long-term tissue-resident macrophages and predominantly exhibit an alternative activation state, whereas cells expressing low-intermediate Cx3CR1eGFP are derived from the BM, contribute to both early and later stages of wound healing, and show both classical and alternative activation states. Diabetic mice showed significant differences in the dynamics of these subpopulations, which likely contribute to elevated and persisting inflammatory states over time. In particular, failure of Cx3CR1int macrophages to mature into Cx3CR1hi links maturation to resolution of inflammation. Thus strategies to promote macrophage maturation may be effective therapeutic tools in chronic inflammatory environments.

Unfolded Protein Response Differentially Regulates TLR4-Induced Cytokine Expression in Distinct Macrophage Populations.

Cellular stress responses are often engaged at sites of inflammation and can alter macrophage cytokine production. We now report that macrophages in distinct states of differentiation or in different temporal stages of inflammatory response exhibit differential sensitivity to cell stress mediated alterations in M1-like polarized inflammatory cytokine production. Tunicamycin (Tm) treatment of bone marrow derived macrophages (BMDM) cultured with M-CSF cultured bone marrow derived macrophages (M-BMDM) had markedly amplified M1-like responses to LPS, exhibiting higher levels of IL12p40 and IL12p35 mRNAs while BMDM cultured with GM-CSF, which normally express high IL12 subunit production in response to LPS, were relatively unaltered. Anti-inflammatory IL10 mRNA production in LPS-stimulated M-BMDM was greatly reduced by cell stress. These changes in cytokine mRNA levels resulted from altered rates of transcription and mRNA decay. Stress also altered cytokine protein production. Resident liver macrophages isolated from mice treated with Tm showed elevated levels of IL12 subunit mRNA production following LPS stimulation. Furthermore, macrophages infiltrating the liver during the early phase of acetaminophen injury (24 h) had little stress-mediated change in cytokine mRNA production while cells isolated in the later phase (48–72 h) exhibited higher sensitivity for stress elevated cytokine production. Hence cultured macrophages developed using different growth/differentiation factors and macrophages from different temporal stages of injury in vivo show markedly different sensitivity to cell stress for altered inflammatory cytokine production. These findings suggest that cellular stress can be an important modulator of the magnitude and character of myeloid inflammatory activity.

Cellular heterogeneity during mouse pancreatic ductal adenocarcinoma progression at single-cell resolution.

e15739 Background: Pancreatic ductal adenocarcinoma (PDA) is a major cause of cancer-related death with limited therapeutic options available. This highlights the need for improved understanding of the biology of PDA progression. The progression of PDA is a highly complex and dynamic process featuring changes in cancer cells and stromal cells; however, a comprehensive characterization of PDA cancer cell and stromal cell heterogeneity during disease progression is lacking. In this study, we aimed to profile cell populations and understand their phenotypic changes during PDA progression. Methods: We employed single-cell RNA sequencing technology to agnostically profile cell heterogeneity during different stages of PDA progression in genetically engineered mouse models. Results: Our data indicate that an epithelial-to-mesenchymal transition of cancer cells accompanies tumor progression. We also found distinct populations of macrophages with increasing inflammatory features during PDA progression. In addition, we noted the existence of three distinct molecular subtypes of fibroblasts in the normal mouse pancreas, which ultimately gave rise to two distinct populations of fibroblasts in advanced PDA, supporting recent reports on intratumoral fibroblast heterogeneity. Our data also suggest that cancer cells and fibroblasts are dynamically regulated by epigenetic mechanisms. Conclusions: This study systematically outlines the landscape of cellular heterogeneity during the progression of PDA. It strongly improves our understanding of the PDA biology and has the potential to aid in the development of therapeutic strategies against specific cell populations of the disease.

Unique Innate Functions of Fetal Macrophage Populations

Abstract In fetal and newborn tissues, macrophages play key roles in generating the innate immune response to pathogens. Distinct macrophage populations arise from hematopoietic precursors in the embryonic yolk sac and fetal liver. Here we tested the hypothesis that fetal macrophage populations possess unique innate immune function based on their cellular origin. Innate immune activation following TLR agonist exposure was tested using FACS sorted fetal macrophage populations or total single cell suspension from fetal lung or fetal liver. TLR agonists increased inflammatory cytokine expression in CD11bHI fetal liver macrophages. However F4/80HI yolk sac derived macrophages failed to respond to TLR activation and expressed very low levels of both Il1b and Il6. Similar data were obtained with ELISA measurement of secreted IL-1beta and IL-6 protein. When total liver cells were treated with TLR agonists, again IL-1beta expression was restricted to CD11bHI macrophages. F4/80HI macrophages from the yolk sac were also less likely to phagocytose labeled E. coli. Expression of multiple TLRs was low or undetectable in F4/80HI macrophages, suggesting a unique overall functional program for this distinct macrophage population. Expression profiling showed similar levels of multiple macrophage signature genes in CD11bHI and F4/80HI cells. However expression of C1q family members and Igf1 were much higher in F4/80HI macrophages. Expression of these genes was more consistent with an anti-inflammatory phenotype and removal of damaged cells during developmental morphogenesis. These important mechanistic differences suggest important cellular and molecular strategies for treating neonatal inflammatory disease.

Translocator protein localises to CD11b+ macrophages in atherosclerosis

Background and aimsAtherosclerosis is characterized by lipid deposition, monocyte infiltration and foam cell formation in the artery wall. Translocator protein (TSPO) is abundantly expressed in lipid rich tissues. Recently, TSPO has been identified as a potential diagnostic tool in cardiovascular disease. The purpose of this study was to determine if the TSPO ligand, 18F-PBR111, can identify early atherosclerotic lesions and if TSPO expression can be used to identify distinct macrophage populations during lesion progression. MethodsApoE−/− mice were maintained on a high-fat diet for 3 or 12 weeks. C57BL/6J mice maintained on chow diet served as controls. Mice were administered 18F-PBR111 intravenously and PET/CT imaged. After euthanasia, aortas were isolated, fixed and optically cleared. Cleared aortas were immunostained with DAPI, and fluorescently labelled with antibodies to TSPO, the tissue resident macrophage marker F4/80 and the monocyte-derived macrophage marker CD11b. TSPO expression and the macrophage markers were visualised in fatty streaks and established plaques by light sheet microscopy. ResultsWhile tissue resident F4/80 + macrophages were evident in the arteries of animals without atherosclerosis, no CD11b + macrophages were observed in these animals. In contrast, established plaques had high CD11b and low F4/80 expression. A ∼3-fold increase in the uptake of 18F-PBR111 was observed in the aortas of atherosclerotic mice relative to controls. ConclusionsImaging of TSPO expression is a new approach for studying atherosclerotic lesion progression and inflammatory cell infiltration. The TSPO ligand, 18F-PBR111, is a potential clinical diagnostic tool for the detection and quantification of atherosclerotic lesion progression in humans.

Two distinct interstitial macrophage populations coexist across tissues in specific subtissular niches.

Macrophages are a heterogeneous cell population involved in tissue homeostasis, inflammation, and various pathologies. Although the major tissue-resident macrophage populations have been extensively studied, interstitial macrophages (IMs) residing within the tissue parenchyma remain poorly defined. Here we studied IMs from murine lung, fat, heart, and dermis. We identified two independent IM subpopulations that are conserved across tissues: Lyve1loMHCIIhiCX3CR1hi (Lyve1loMHCIIhi) and Lyve1hiMHCIIloCX3CR1lo (Lyve1hiMHCIIlo) monocyte-derived IMs, with distinct gene expression profiles, phenotypes, functions, and localizations. Using a new mouse model of inducible macrophage depletion (Slco2b1 flox/DTR), we found that the absence of Lyve1hiMHCIIlo IMs exacerbated experimental lung fibrosis. Thus, we demonstrate that two independent populations of IMs coexist across tissues and exhibit conserved niche-dependent functional programming.

Macrophages in obesity and non-alcoholic fatty liver disease: Crosstalk with metabolism.

SummaryNon-alcoholic fatty liver disease (NAFLD) is the most prevalent liver disease worldwide, and a major cause of liver cirrhosis and hepatocellular carcinoma. NAFLD is intimately linked with other metabolic disorders characterized by insulin resistance. Metabolic diseases are driven by chronic inflammatory processes, in which macrophages perform essential roles. The polarization status of macrophages is itself influenced by metabolic stimuli such as fatty acids, which in turn affect the progression of metabolic dysfunction at multiple disease stages and in various tissues. For instance, adipose tissue macrophages respond to obesity, adipocyte stress and dietary factors by a specific metabolic and inflammatory programme that stimulates disease progression locally and in the liver. Kupffer cells and monocyte-derived macrophages represent ontologically distinct hepatic macrophage populations that perform a range of metabolic functions. These macrophages integrate signals from the gut-liver axis (related to dysbiosis, reduced intestinal barrier integrity, endotoxemia), from overnutrition, from systemic low-grade inflammation and from the local environment of a steatotic liver. This makes them central players in the progression of NAFLD to steatohepatitis (non-alcoholic steatohepatitis or NASH) and fibrosis. Moreover, the particular involvement of Kupffer cells in lipid metabolism, as well as the inflammatory activation of hepatic macrophages, may pathogenically link NAFLD/NASH and cardiovascular disease. In this review, we highlight the polarization, classification and function of macrophage subsets and their interaction with metabolic cues in the pathophysiology of obesity and NAFLD. Evidence from animal and clinical studies suggests that macrophage targeting may improve the course of NAFLD and related metabolic disorders.