Differentiation and heterogeneity in the mononuclear phagocyte system

Comprehensive analysis of TLR4-induced transcriptional responses in interleukin 4-primed mouse macrophages

Interferon (IFN)gamma and interleukin (IL)-4 are central regulators of T helper 1 (Th1) and T helper 2 (Th2) immune responses, respectively. Both cytokines have a major impact on macrophage phenotypes: IFNgamma-priming and subsequent TLR4 activation induces so-called "classically activated" macrophages that are characterized by pronounced pro-inflammatory responses, whereas IL-4-treated macrophages, commonly called "alternatively activated", are known to develop enhanced capacity for endocytosis, antigen presentation and tissue repair and are generally considered anti-inflammatory. Considering IL-4 as priming rather than activating stimulus, we now compared the TLR4-dependent global gene activation program in IFNgamma- versus IL-4-pretreated mouse macrophages, which has rarely been studied so far. Although both cytokines frequently induced opposing effects on gene transcription, the subsequent activation of bone marrow-derived macrophages by lipopolysaccharide (LPS) produced a strong, priming-dependent pro-inflammatory response in both macrophage types. For example, the production of key pro-inflammatory cytokines IL-6 and IL-12 was significantly higher in IL-4- versus IFNgamma-primed macrophages and several cytokine genes, including Il19, Ccl17, Ccl22, Ccl24 and Cxcl5, were preferentially induced in "alternatively" primed and LPS activated mouse macrophages. In a subset of genes, including IL12a, IFNgamma-priming was actually found to suppress LPS-induced gene expression in a Stat1-dependent manner. Our data suggest that IL-4-priming is not per se anti-inflammatory but generates a macrophage that is "tissue protective" but still capable of mounting a strong inflammatory response after TLR4-dependent activation.

Meta-analysis of lineage-specific gene expression signatures in mouse leukocyte populations

In order to address fundamental questions associated with the relationships between mononuclear phagocytes and other myeloid and lymphoid cell populations, we have taken advantage of the growing body of expression data available in the public domain. We collated a large number of published expression studies on mouse haemopoietic cell lineages comprising 304 cell samples from 29 independent experiments performed on a single microarray platform (Affymetrix MOE430-2). The data were subjected to network-based cluster analysis using Biolayout Express(3D). Genes with related function clustered together in distinct regions of the graph reaffirming many known associations between gene expression and role in specific pathways and defining most major cell types of the immune system. Promoters of genes within individual clusters were distinguished by over-representation of regulatory motifs recognised by specific transcription factors. However, these data indicate that commonly used myeloid subpopulation markers, such as CD11c (Itgax), do not correlate with expression of other genes, and further bring into question their use in defining myeloid cell lineage, activation (M1 vs. M2) and antigen-presenting cell function. In particular, there were few mRNA markers that clearly distinguished classical dendritic cells (DC) from macrophages, other than low expression of genes required for phagocytic activity. Bone marrow-derived DC, grown in GM-CSF, were clearly identified as phagocytes and distinguished from isolated lymphoid tissue DC. Thus, through pooling datasets from public data and examining the gene expression clusters within, we can learn a great deal about the transcriptional networks that underpin the differences in functional activities between cell populations of the immune system.

Cellular and molecular mechanisms of bone remodeling

Physiological bone remodeling is a highly coordinated process responsible for bone resorption and formation and is necessary to repair damaged bone and to maintain mineral homeostasis. In addition to the traditional bone cells (osteoclasts, osteoblasts, and osteocytes) that are necessary for bone remodeling, several immune cells have also been implicated in bone disease. This minireview discusses physiological bone remodeling, outlining the traditional bone biology dogma in light of emerging osteoimmunology data. Specifically discussed in detail are the cellular and molecular mechanisms of bone remodeling, including events that orchestrate the five sequential phases of bone remodeling: activation, resorption, reversal, formation, and termination.

Induction of fatty acid synthesis is a key requirement for phagocytic differentiation of human monocytes

Monocytes are precursors of macrophages. Here we demonstrate that macrophage colony-stimulating factor (M-CSF)-dependent differentiation of primary human monocytes from healthy volunteers induces transcription of SREBP-1c target genes required for fatty acid (FA) biosynthesis and impairs transcription of SREBP-2 target genes required for cholesterol synthesis. Detailed lipid metabolic profiling showed that this transcriptional regulation leads to a dramatically increased fatty acid synthesis as driving force for enhanced phospholipid synthesis. During cell differentiation the major lipid class switches from cholesterol in monocytes to phosphatidylcholine in macrophages. Ultrastructural analysis revealed that this transcriptional and metabolic regulation is essential for development of macrophage filopodia and cellular organelles including primary lysosomes, endoplasmic reticulum, and Golgi network. Additional functional studies showed that suppression of fatty acid synthesis prevents phagocytosis representing a central macrophage function. Therefore induction of fatty acid synthesis is a key requirement for phagocyte development and function.

Functional clustering and lineage markers: insights into cellular differentiation and gene function from large-scale microarray studies of purified primary cell populations

Very large microarray datasets showing gene expression across multiple tissues and cell populations provide a window on the transcriptional networks that underpin the differences in functional activity between biological systems. Clusters of co-expressed genes provide lineage markers, candidate regulators of cell function and, by applying the principle of guilt by association, candidate functions for genes of currently unknown function. We have analysed a dataset comprising pure cell populations from hemopoietic and non-hemopoietic cell types (http://biogps.gnf.org). Using a novel network visualisation and clustering approach, we demonstrate that it is possible to identify very tight expression signatures associated specifically with embryonic stem cells, mesenchymal cells and hematopoietic lineages. Selected examples validate the prediction that gene function can be inferred by co-expression. One expression cluster was enriched in phagocytes, which, alongside endosome-lysosome constituents, contains genes that may make up a 'pathway' for phagocyte differentiation. Promoters of these genes are enriched for binding sites for the ETS/PU.1 and MITF families. Another cluster was associated with the production of a specific extracellular matrix, with high levels of gene expression shared by cells of mesenchymal origin (fibroblasts, adipocytes, osteoblasts and myoblasts). We discuss the limitations placed upon such data by the presence of alternative promoters with distinct tissue specificity within many protein-coding genes.

Luteolin triggers global changes in the microglial transcriptome leading to a unique anti-inflammatory and neuroprotective phenotype

Background

Luteolin, a plant derived flavonoid, exerts a variety of pharmacological activities and anti-oxidant properties associated with its capacity to scavenge oxygen and nitrogen species. Luteolin also shows potent anti-inflammatory activities by inhibiting nuclear factor kappa B (NFkB) signaling in immune cells. To better understand the immuno-modulatory effects of this important flavonoid, we performed a genome-wide expression analysis in pro-inflammatory challenged microglia treated with luteolin and conducted a phenotypic and functional characterization.

Methods

Resting and LPS-activated BV-2 microglia were treated with luteolin in various concentrations and mRNA levels of pro-inflammatory markers were determined. DNA microarray experiments and bioinformatic data mining were performed to capture global transcriptomic changes following luteolin stimulation of microglia. Extensive qRT-PCR analyses were carried out for an independent confirmation of newly identified luteolin-regulated transcripts. The activation state of luteolin-treated microglia was assessed by morphological characterization. Microglia-mediated neurotoxicity was assessed by quantifying secreted nitric oxide levels and apoptosis of 661W photoreceptors cultured in microglia-conditioned medium.

Results

Luteolin dose-dependently suppressed pro-inflammatory marker expression in LPS-activated microglia and triggered global changes in the microglial transcriptome with more than 50 differentially expressed transcripts. Pro-inflammatory and pro-apoptotic gene expression was effectively blocked by luteolin. In contrast, mRNA levels of genes related to anti-oxidant metabolism, phagocytic uptake, ramification, and chemotaxis were significantly induced. Luteolin treatment had a major effect on microglial morphology leading to ramification of formerly amoeboid cells associated with the formation of long filopodia. When co-incubated with luteolin, LPS-activated microglia showed strongly reduced NO secretion and significantly decreased neurotoxicity on 661W photoreceptor cultures.

Conclusions

Our findings confirm the inhibitory effects of luteolin on pro-inflammatory cytokine expression in microglia. Moreover, our transcriptomic data suggest that this flavonoid is a potent modulator of microglial activation and affects several signaling pathways leading to a unique phenotype with anti-inflammatory, anti-oxidative, and neuroprotective characteristics. With the identification of several novel luteolin-regulated genes, our findings provide a molecular basis to understand the versatile effects of luteolin on microglial homeostasis. The data also suggest that luteolin could be a promising candidate to develop immuno-modulatory and neuroprotective therapies for the treatment of neurodegenerative disorders.

Cellular plasticity of inflammatory myeloid cells in the peritoneal foreign body response

Implantation of sterile foreign objects in the peritoneal cavity of an animal initiates an inflammatory response and results in encapsulation of the objects by bone marrow-derived cells. Over time, a multilayered tissue capsule develops with abundant myofibroblasts embedded in extracellular matrix. The present study used the transgenic MacGreen mouse to characterize the time-dependent accumulation of monocyte subsets and neutrophilic granulocytes in the inflammatory infiltrate and within the tissue capsule by their differential expression of the csf1r-EGFP transgene, F4/80, and Ly6C. As the tissue capsule developed, enhanced green fluorescent protein-positive cells changed from rounded to spindle-shaped morphology and began to co-express the myofibroblast marker alpha-smooth muscle actin. Expression increased with time: at day 14, 11.13 +/- 0.67% of tissue capsule cells co-expressed these markers, compared with 50.77 +/- 12.85% of cells at day 28. The importance of monocyte/macrophages in tissue capsule development was confirmed by clodronate-encapsulated liposome removal, which resulted in almost complete abrogation of capsule development. These results confirm the importance of monocyte/macrophages in the tissue response to sterile foreign objects implanted in the peritoneal cavity. In addition, the in vivo plasticity of peritoneal macrophages and their ability to transdifferentiate from a myeloid to mesenchymal phenotype is demonstrated.

Alterations in cellular proteome and secretome upon differentiation from monocyte to macrophage by treatment with phorbol myristate acetate: insights into biological processes

Monocyte and macrophage are mainly involved in immune response and inflammatory processes. Monocytes circulate in the bloodstream and migrate to various tissues where they can differentiate to macrophages. However, the molecular basis of biological processes involved in this cellular differentiation remains ambiguous. This study was to investigate alterations in cellular and secreted proteins after this differentiation phase. Macrophage was differentiated from U937 human monocytic cell line by treatment with 100 ng/ml phorbol myristate acetate (PMA) for 48 h. Cellular and secreted proteins extracted from PMA-treated cells (macrophages) were compared with those of untreated cells (monocytes) using 2-DE (n=5 gels/condition; stained with Deep Purple fluorescence dye). Quantitative intensity analysis revealed 81 and 67 protein spots whose levels were significantly altered in cellular proteome and secretome. These proteins were subsequently identified by Q-TOF MS and/or MS/MS analyses. The altered levels of cellular elongation factor-2 (EF-2) and secreted alpha-tubulin were confirmed by Western blot analysis. Global protein network analysis demonstrated that these altered proteins were involved in cell death, lipid metabolism, cell morphology, cellular movement, and protein folding. Our data may provide some insights into molecular mechanisms of biological processes upon differentiation from monocytes to macrophages.

CSF-1 signals directly to renal tubular epithelial cells to mediate repair in mice

Tubular damage following ischemic renal injury is often reversible, and tubular epithelial cell (TEC) proliferation is a hallmark of tubular repair. Macrophages have been implicated in tissue repair, and CSF-1, the principal macrophage growth factor, is expressed by TECs. We therefore tested the hypothesis that CSF-1 is central to tubular repair using an acute renal injury and repair model, ischemia/reperfusion (I/R). Mice injected with CSF-1 following I/R exhibited hastened healing, as evidenced by decreased tubular pathology, reduced fibrosis, and improved renal function. Notably, CSF-1 treatment increased TEC proliferation and reduced TEC apoptosis. Moreover, administration of a CSF-1 receptor-specific (CSF-1R-specific) antibody after I/R increased tubular pathology and fibrosis, suppressed TEC proliferation, and heightened TEC apoptosis. To determine the contribution of macrophages to CSF-1-dependent renal repair, we assessed the effect of CSF-1 on I/R in mice in which CD11b+ cells were genetically ablated and determined that macrophages only partially accounted for CSF-1-dependent tubular repair. We found that TECs expressed the CSF-1R and that this receptor was upregulated and coexpressed with CSF-1 in TECs following renal injury in mice and humans. Furthermore, signaling via the CSF-1R stimulated proliferation and reduced apoptosis in human and mouse TECs. Taken together, these data suggest that CSF-1 mediates renal repair by both a macrophage-dependent mechanism and direct autocrine/paracrine action on TECs.

Intestinal macrophages: differentiation and involvement in intestinal immunopathologies

Intestinal macrophages, preferentially located in the subepithelial lamina propria, represent the largest pool of tissue macrophages in humans. As an adaptation to the local antigen- and bacteria-rich environment, intestinal macrophages exhibit several distinct phenotypic and functional characteristics. Notably, microbe-associated molecular pattern receptors, including the lipopolysaccharide (LPS) receptors CD14 and TLR4, and also the Fc receptors for IgA and IgG are absent on most intestinal macrophages under homeostatic conditions. Moreover, while macrophages in the intestinal mucosa are refractory to the induction of proinflammatory cytokine secretion, they still display potent phagocytic activity. These adaptations allow intestinal macrophages to comply with their main task, i.e., the efficient removal of microbes while maintaining local tissue homeostasis. In this paper, we review recent findings on the functional differentiation of monocyte subsets into distinct macrophage populations and on the phenotypic and functional adaptations that have evolved in intestinal macrophages in response to their antigen-rich environment. Furthermore, the involvement of intestinal macrophages in the pathogenesis of celiac disease and inflammatory bowel diseases is discussed.