Structural basis for the dual recognition of helical cytokines IL-34 and CSF-1 by CSF-1R

IL-34 is associated with obesity, chronic inflammation, and insulin resistance

OBJECTIVES

IL-34 is a recently identified alternative ligand for colony-stimulating factor-1 (CSF-1) receptor. IL-34 and CSF-1 are regulators of differentiation, proliferation, and survival in mononuclear phagocytes. Here, we investigated the IL-34 serum concentration and expression in human adipose tissues and any associations with insulin resistance.

METHODS

We recruited 19 nondiabetic obese women, 9 type 2 diabetic women, and 27 normal-weight women. Metabolic parameters, abdominal fat distribution, serum IL-34 concentration, and IL-34 mRNA expression were measured in abdominal sc adipose tissue (SAT) and visceral adipose tissue (VAT). In addition, the expression/secretion and putative effects of IL-34 were assessed in human differentiated adipocytes. Serum IL-34 concentration was measured before and 5 to 9 months after laparoscopic Roux-en-Y gastric bypass surgery was performed on the 20 obese patients.

RESULTS

Regardless of diabetes status, obese patients demonstrated significantly higher serum IL-34 concentrations than controls. Serum IL-34 was significantly and positively correlated with insulin resistance-related metabolic parameters. IL-34 mRNA was significantly higher in VAT than SAT. IL-34 was expressed in adipocytes as well as nonadipocytes, and expression was significantly higher during adipogenesis. In differentiated adipocytes, the expression/secretion of IL-34 was enhanced by TNFα and IL-1β. In addition, IL-34 augmented fat accumulation and inhibited the stimulatory effects of insulin on glucose transport. Moreover, serum IL-34 was significantly decreased after Roux-en-Y gastric bypass-induced weight loss.

CONCLUSION

The present study demonstrates, for the first time, that IL-34 is expressed in human adipose tissues and the circulating concentration is significantly elevated in obese patients. This suggests that IL-34 is associated with insulin resistance.

Targeting tumor-associated macrophages with anti-CSF-1R antibody reveals a strategy for cancer therapy

Macrophage infiltration has been identified as an independent poor prognostic factor in several cancer types. The major survival factor for these macrophages is macrophage colony-stimulating factor 1 (CSF-1). We generated a monoclonal antibody (RG7155) that inhibits CSF-1 receptor (CSF-1R) activation. In vitro RG7155 treatment results in cell death of CSF-1-differentiated macrophages. In animal models, CSF-1R inhibition strongly reduces F4/80(+) tumor-associated macrophages accompanied by an increase of the CD8(+)/CD4(+) T cell ratio. Administration of RG7155 to patients led to striking reductions of CSF-1R(+)CD163(+) macrophages in tumor tissues, which translated into clinical objective responses in diffuse-type giant cell tumor (Dt-GCT) patients.

Monocyte homeostasis and the plasticity of inflammatory monocytes

Monocytes are mononuclear myeloid cells that develop in the bone marrow and circulate within the bloodstream. Although they have long been argued to play a role in the repopulation of tissue-resident macrophages, this has been questioned by numerous recent studies, which has forced a reappraisal of their biology. Here we discuss monocyte development, as well as the homeostatic control of monocyte subpopulations within the blood. We also outline the known functions of monocyte subsets. Finally, we highlight the plastic nature of monocytes, which are capable of a remarkable range of phenotypic and functional changes that depend on signals from local microenvironments.

Targeting IL-34 in chronic inflammation

A second ligand for colony-stimulating factor-1 receptor (CSF-1R) with distinct biologic activities had long been implicated but not appreciated until the recent discovery of interleukin (IL)-34. IL-34 and CSF-1 signal through this common receptor to mediate the biology of mononuclear phagocytic cells. Aberrant macrophage activation by CSF-1 and/or IL-34 is associated with numerous diseases, and clinical therapies targeting this pathway are being tested. Although IL-34 and CSF-1 have distinct activities under physiologic conditions, they appear functionally redundant in various disease states. Thus, blocking the activity of both might be necessary for maximal efficacy.

CSF-1 receptor signaling in myeloid cells

The CSF-1 receptor (CSF-1R) is activated by the homodimeric growth factors colony-stimulating factor-1 (CSF-1) and interleukin-34 (IL-34). It plays important roles in development and in innate immunity by regulating the development of most tissue macrophages and osteoclasts, of Langerhans cells of the skin, of Paneth cells of the small intestine, and of brain microglia. It also regulates the differentiation of neural progenitor cells and controls functions of oocytes and trophoblastic cells in the female reproductive tract. Owing to this broad tissue expression pattern, it plays a central role in neoplastic, inflammatory, and neurological diseases. In this review we summarize the evolution, structure, and regulation of expression of the CSF-1R gene. We discuss the structures of CSF-1, IL-34, and the CSF-1R and the mechanism of ligand binding to and activation of the receptor. We further describe the pathways regulating macrophage survival, proliferation, differentiation, and chemotaxis downstream from the CSF-1R.

Interkeukin-34, a cytokine crucial for the differentiation and maintenance of tissue resident macrophages and Langerhans cells

IL-34 is a recently discovered cytokine that acts on tissue resident macrophages and Langerhans cells upon binding the receptor for CSF-1, CSF-1R. The existence of two ligands for CSF-1R, IL-34, and CSF-1, raises several intriguing questions. Are IL-34 and CSF-1 redundant or does each perform temporally and spatially distinct functions? Is IL-34 involved in human pathology? Would therapeutic strategies based on selective inhibition or administration of either IL-34 or CSF-1 be advantageous for preventing human pathology? Recent in vivo studies indicate that IL-34 promotes the development, survival, and function of microglia and Langerhans cells; therefore, this cytokine may predominately function in brain and skin biology. Here, we review the evidence for IL-34 as a key cytokine in the development and function of these two diverse cell types and discuss its potential role in pathological conditions.

Challenging the state of the art in protein structure prediction: Highlights of experimental target structures for the 10th Critical Assessment of Techniques for Protein Structure Prediction Experiment CASP10

For the last two decades, CASP has assessed the state of the art in techniques for protein structure prediction and identified areas which required further development. CASP would not have been possible without the prediction targets provided by the experimental structural biology community. In the latest experiment, CASP10, more than 100 structures were suggested as prediction targets, some of which appeared to be extraordinarily difficult for modeling. In this article, authors of some of the most challenging targets discuss which specific scientific question motivated the experimental structure determination of the target protein, which structural features were especially interesting from a structural or functional perspective, and to what extent these features were correctly reproduced in the predictions submitted to CASP10. Specifically, the following targets will be presented: the acid-gated urea channel, a difficult to predict transmembrane protein from the important human pathogen Helicobacter pylori; the structure of human interleukin (IL)-34, a recently discovered helical cytokine; the structure of a functionally uncharacterized enzyme OrfY from Thermoproteus tenax formed by a gene duplication and a novel fold; an ORFan domain of mimivirus sulfhydryl oxidase R596; the fiber protein gene product 17 from bacteriophage T7; the bacteriophage CBA-120 tailspike protein; a virus coat protein from metagenomic samples of the marine environment; and finally, an unprecedented class of structure prediction targets based on engineered disulfide-rich small proteins.

Production and characterisation of a monoclonal antibody that recognises the chicken CSF1 receptor and confirms that expression is restricted to macrophage-lineage cells

Macrophages contribute to innate and acquired immunity as well as many aspects of homeostasis and development. Studies of macrophage biology and function in birds have been hampered by a lack of definitive cell surface markers. As in mammals, avian macrophages proliferate and differentiate in response to CSF1 and IL34, acting through the shared receptor, CSF1R. CSF1R mRNA expression in the chicken is restricted to macrophages and their progenitors. To expedite studies of avian macrophage biology, we produced an avian CSF1R-Fc chimeric protein and generated a monoclonal antibody (designated ROS-AV170) against the chicken CSF1R using the chimeric protein as immunogen. Specific binding of ROS-AV170 to CSF1R was confirmed by FACS, ELISA and immunohistochemistry on tissue sections. CSF1 down-regulated cell surface expression of the CSF1R detected with ROS-AV170, but the antibody did not block CSF1 signalling. Expression of CSF1R was detected on the surface of bone marrow progenitors only after culture in the absence of CSF1, and was induced during macrophage differentiation. Constitutive surface expression of CSF1R distinguished monocytes from other myeloid cells, including heterophils and thrombocytes. This antibody will therefore be of considerable utility for the study of chicken macrophage biology.

3D modeling and characterization of the human CD115 monoclonal antibody H27K15 epitope and design of a chimeric CD115 target

The humanized monoclonal antibody H27K15 specifically targets human CD115, a type III tyrosine kinase receptor involved in multiple cancers and inflammatory diseases. Binding of H27K15 to hCD115 expressing cells inhibits the functional effect of colony-stimulating factor-1 (CSF-1), in a non-competitive manner. Both homology modeling and docking programs were used here to model the human CD115 extracellular domains, the H27K15 variable region and their interaction. The resulting predicted H27K15 epitope includes mainly the D1 domain in the N-terminal extracellular region of CD115 and some residues of the D2 domain. Sequence alignment with the non-binding murine CD115, enzyme-linked immunosorbent assay, nuclear magnetic resonance spectroscopy and affinity measurements by quartz crystal microbalance revealed critical residues of this epitope that are essential for H27K15 binding. A combination of computational simulations and biochemical experiments led to the design of a chimeric CD115 carrying the human epitope of H27K15 in a murine CD115 backbone that is able to bind both H27K15 as well as the murine ligands CSF-1 and IL-34. These results provide new possibilities to minutely study the functional effects of H27K15 in a transgenic mouse that would express this chimeric molecule.

Alterations of the interstitial cells of Cajal and the microstructure of the gastrointestinal tract in KIT distal kinase mutant mice

The development and maintenance of interstitial cells of Cajal (ICC) are closely associated with SCF/KIT signal activity. In this study, we evaluate the distribution of ICC in KIT distal kinase domain mutant mice (Wads) and determine whether the loss-of-function mutations in KIT easily lead to gastrointestinal (GI) disorders. ICC were examined by anti-KIT immunohistochemistry and western blotting. The GI microstructure of wild-type (WT) and Wads mice in normal intestines and incomplete intestinal obstruction was evaluated by hematoxylin and eosin staining. The results in Wads(m/m) mice were as follows. Myenteric ICC were obviously decreased in the stomach and colon and were totally absent in the small intestine. Intramuscular ICC were nearly absent in the stomach and irregularly distributed in the colon. Moreover, the smooth muscle thickness of the small intestine was increased 1.3-fold in Wads(m/m), compared to WT and Wads(m/+) mice and the diameter of the intestinal lumen was also enlarged in Wads(m/m) mice. When constructing an incomplete intestinal obstruction model, the extent of distention involved was greater in Wads mice (1.6-fold in Wads(m/+) mice and 1.8-fold in Wads(m/m) mice vs. WT mice). Meanwhile, the intestinal lumen expansion and decrease in ICC were more pronounced in Wads mice than in WT mice. Our results suggest that the KIT distal kinase domain mutation leads to an ICC loss in a subtype and location-specific pattern in Wads(m/m) mice. The injury of the KIT signaling in mutant mice results in more serious pathological manifestations after being exposed to pathogenic factors.

IL-34 and CSF-1: similarities and differences

Colony-stimulating factor-1 (CSF-1) is widely expressed and considered to regulate the development, maintenance, and function of mononuclear phagocyte lineage cells such as monocytes, macrophages, dendritic cells (DCs), Langerhans cells (LCs), microglia, and osteoclasts. Interleukin-34 (IL-34) was recently identified as an alternative ligand for the CSF-1 receptor (CSF-1R) through functional proteomics experiments. It is well established that the phenotype of CSF-1R-deficient (CSF-1R⁻/⁻) mice is more severe than that of mice bearing a spontaneous null mutation in CSF-1 (CSF-1(op/op)). CSF-1R⁻/⁻ mice are severely depleted of macrophages and completely lack LCs, microglia, and osteoclasts during their lifetime. In contrast, CSF-1(op/op) mice exhibit late-onset macrophage development and osteoclastogenesis, whereas they show modestly reduced numbers of microglia and a relatively normal LC development. In contrast, IL-34-deficient (IL-34⁻/⁻) mice show a marked reduction of LCs and a decrease in microglia. IL-34 and CSF-1 display different spatiotemporal expression patterns and have distinct biological functions. In this review, we focus on the functional similarities and differences between IL-34 and CSF-1 in vivo.

Macrophages and CSF-1: implications for development and beyond

Recent focus on the diversity of macrophage phenotype and function signifies that these trophic cells are no longer of exclusive interest to the field of immunology. As key orchestrators of organogenesis, the contribution of macrophages to fetal development is worthy of greater attention. This review summarizes the key functions of macrophages and their primary regulator, colony-stimulating factor (CSF)-1, during development; highlighting trophic mechanisms beyond phagocytosis and outlining their roles in a range of developing organ systems. Advances in the understanding of macrophage polarization and functional heterogeneity are discussed from a developmental perspective. In addition, this review highlights the relevance of CSF-1 as a pleiotropic developmental growth factor and summarizes recent experimental evidence and clinical advancements in the area of CSF-1 and macrophage manipulation in reproduction and organogenic settings. Interrogation of embryonic macrophages also has implications beyond development, with recent attention focused on yolk sac macrophage ontogeny and their role in homeostasis and mediating tissue regeneration. The regulatory networks that govern development involve a complex range of growth factors, signaling pathways and transcriptional regulators arising from epithelial, mesenchymal and stromal origins. A component of the organogenic milieu common to the majority of developing organs is the tissue macrophage. These hemopoietic cells are part of the mononuclear phagocyte system regulated primarily by colony-stimulating factor (CSF)-1 (1, 2). There is a resurgence in the field of CSF-1 and macrophage biology; where greater understanding of the heterogeneity of these cells is revealing contributions to tissue repair and regeneration beyond the phagocytic and inflammatory functions for which they were traditionally ascribed (3-6). The accumulation of macrophages during tissue injury is no longer viewed as simply a surrogate for disease severity, with macrophages now known to be vital in governing tissue regeneration in many settings (7-11). In particular it is the influence of CSF-1 in regulating an alternative macrophage activation state that is increasingly linked to organ repair in a range of disease models (12-17). With many similarities drawn between organogenesis and regeneration, it is pertinent to re-examine the role of CSF-1 and macrophages in organ development.

Structural and mechanistic insights into VEGF receptor 3 ligand binding and activation

Vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) are key drivers of blood and lymph vessel formation in development, but also in several pathological processes. VEGF-C signaling through VEGFR-3 promotes lymphangiogenesis, which is a clinically relevant target for treating lymphatic insufficiency and for blocking tumor angiogenesis and metastasis. The extracellular domain of VEGFRs consists of seven Ig homology domains; domains 1-3 (D1-3) are responsible for ligand binding, and the membrane-proximal domains 4-7 (D4-7) are involved in structural rearrangements essential for receptor dimerization and activation. Here we analyzed the crystal structures of VEGF-C in complex with VEGFR-3 domains D1-2 and of the VEGFR-3 D4-5 homodimer. The structures revealed a conserved ligand-binding interface in D2 and a unique mechanism for VEGFR dimerization and activation, with homotypic interactions in D5. Mutation of the conserved residues mediating the D5 interaction (Thr446 and Lys516) and the D7 interaction (Arg737) compromised VEGF-C induced VEGFR-3 activation. A thermodynamic analysis of VEGFR-3 deletion mutants showed that D3, D4-5, and D6-7 all contribute to ligand binding. A structural model of the VEGF-C/VEGFR-3 D1-7 complex derived from small-angle X-ray scattering data is consistent with the homotypic interactions in D5 and D7. Taken together, our data show that ligand-dependent homotypic interactions in D5 and D7 are essential for VEGFR activation, opening promising possibilities for the design of VEGFR-specific drugs.

Receptor-type protein-tyrosine phosphatase ζ is a functional receptor for interleukin-34

Interleukin-34 (IL-34) is highly expressed in brain. IL-34 signaling via its cognate receptor, colony-stimulating factor-1 receptor (CSF-1R), is required for the development of microglia. However, the differential expression of IL-34 and the CSF-1R in brain suggests that IL-34 may signal via an alternate receptor. By IL-34 affinity chromatography of solubilized mouse brain membrane followed by mass spectrometric analysis, we identified receptor-type protein-tyrosine phosphatase ζ (PTP-ζ), a cell surface chondroitin sulfate (CS) proteoglycan, as a novel IL-34 receptor. PTP-ζ is primarily expressed on neural progenitors and glial cells and is highly expressed in human glioblastomas. IL-34 selectively bound PTP-ζ in CSF-1R-deficient U251 human glioblastoma cell lysates and inhibited the proliferation, clonogenicity, and motility of U251 cells in a PTP-ζ-dependent manner. These effects were correlated with an increase in tyrosine phosphorylation of the previously identified PTP-ζ downstream effectors focal adhesion kinase and paxillin. IL-34 binding to U251 cells was abrogated by chondroitinase ABC treatment, and CS competed with IL-34 for binding to the extracellular domain of PTP-ζ and to the cells, indicating a dependence of binding on PTP-ζ CS moieties. This study identifies an alternate receptor for IL-34 that may mediate its action on novel cellular targets.

Design and pharmacology of a highly specific dual FMS and KIT kinase inhibitor

Inflammation and cancer, two therapeutic areas historically addressed by separate drug discovery efforts, are now coupled in treatment approaches by a growing understanding of the dynamic molecular dialogues between immune and cancer cells. Agents that target specific compartments of the immune system, therefore, not only bring new disease modifying modalities to inflammatory diseases, but also offer a new avenue to cancer therapy by disrupting immune components of the microenvironment that foster tumor growth, progression, immune evasion, and treatment resistance. McDonough feline sarcoma viral (v-fms) oncogene homolog (FMS) and v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog (KIT) are two hematopoietic cell surface receptors that regulate the development and function of macrophages and mast cells, respectively. We disclose a highly specific dual FMS and KIT kinase inhibitor developed from a multifaceted chemical scaffold. As expected, this inhibitor blocks the activation of macrophages, osteoclasts, and mast cells controlled by these two receptors. More importantly, the dual FMS and KIT inhibition profile has translated into a combination of benefits in preclinical disease models of inflammation and cancer.

Human IL-34 and CSF-1 establish structurally similar extracellular assemblies with their common hematopoietic receptor

The discovery that hematopoietic human colony stimulating factor-1 receptor (CSF-1R) can be activated by two distinct cognate cytokines, colony stimulating factor-1 (CSF-1) and interleukin-34 (IL-34), created puzzling scenarios for the two possible signaling complexes. We here employ a hybrid structural approach based on small-angle X-ray scattering (SAXS) and negative-stain EM to reveal that bivalent binding of human IL-34 to CSF-1R leads to an extracellular assembly hallmarked by striking similarities to the CSF-1:CSF-1R complex, including homotypic receptor-receptor interactions. Thus, IL-34 and CSF-1 have evolved to exploit the geometric requirements of CSF-1R activation. Our models include N-linked oligomannose glycans derived from a systematic approach resulting in the accurate fitting of glycosylated models to the SAXS data. We further show that the C-terminal region of IL-34 is heavily glycosylated and that it can be proteolytically cleaved from the IL-34:hCSF-1R complex, providing insights into its role in the functional nonredundancy of IL-34 and CSF-1.

IL-34 induces the differentiation of human monocytes into immunosuppressive macrophages. antagonistic effects of GM-CSF and IFNγ

IL-34 is a recently identified cytokine that signals via the M-CSF receptor and promotes monocyte survival. Depending on the environment, monocytes can differentiate into macrophages (Mφ) or dendritic cells (DC). A wide spectrum of Mφ and DC subsets, with distinct phenotypes and functions, has been described. To date, the phenotype of monocytes exposed to IL-34 remains unexplored. We report here that IL-34 induces the differentiation of monocytes into CD14^high CD163^high CD1a⁻ Mφ (IL-34-Mφ). Upon LPS stimulation, IL-34-Mφ exhibit an IL-10^high IL-12^low M2 profile and express low levels of the costimulatory molecules CD80 and CD86. IL-34-Mφ exhibit poor T cell costimulatory properties, and have potent immunosuppressive properties (decrease of TCR-stimulated T cell proliferation). For all the parameters analyzed, IL-34-Mφ are phenotypically and functionally similar to M-CSF-Mφ. IL-34 appears as efficient as M-CSF in inducing the generation of immunosuppressive Mφ. Moreover, the generation of IL-34-Mφ is mediated through the M-CSF receptor, is independent of endogenous M-CSF consumption and is potentiated by IL-6. In an attempt to identify strategies to prevent a deleterious M2 cell accumulation in some pathological situations, we observed that IFNγ and GM-CSF prevent the generation of immunosuppressive Mφ induced by IL-34. IFNγ also switches established IL-34-Mφ into immunostimulatory Mφ. In conclusion, we demonstrate that IL-34 drives the differentiation of monocytes into immunosuppressive M2, in a manner similar to M-CSF, and that IFNγ and GM-CSF prevent this effect.

Cloning and expression of feline colony stimulating factor receptor (CSF-1R) and analysis of the species specificity of stimulation by colony stimulating factor-1 (CSF-1) and interleukin-34 (IL-34)

Colony stimulating factor (CSF-1) and its receptor, CSF-1R, have been previously well studied in humans and rodents to dissect the role they play in development of cells of the mononuclear phagocyte system. A second ligand for the CSF-1R, IL-34 has been described in several species. In this study, we have cloned and expressed the feline CSF-1R and examined the responsiveness to CSF-1 and IL-34 from a range of species. The results indicate that pig and human CSF-1 and human IL-34 are equally effective in cats, where both mouse CSF-1 and IL-34 are significantly less active. Recombinant human CSF-1 can be used to generate populations of feline bone marrow and monocyte derived macrophages that can be used to further dissect macrophage-specific gene expression in this species, and to compare it to data derived from mouse, human and pig. These results set the scene for therapeutic use of CSF-1 and IL-34 in cats.

Cloning and expression of porcine Colony Stimulating Factor-1 (CSF-1) and Colony Stimulating Factor-1 Receptor (CSF-1R) and analysis of the species specificity of stimulation by CSF-1 and Interleukin 34

Macrophage Colony Stimulating Factor (CSF-1) controls the survival, differentiation and proliferation of cells of the mononuclear phagocyte system. A second ligand for the CSF-1R, Interleukin 34 (IL-34), has been described, but its physiological role is not yet known. The domestic pig provides an alternative to traditional rodent models for evaluating potential therapeutic applications of CSF-1R agonists and antagonists. To enable such studies, we cloned and expressed active pig CSF-1. To provide a bioassay, pig CSF-1R was expressed in the factor-dependent Ba/F3 cell line. On this transfected cell line, recombinant porcine CSF-1 and human CSF-1 had identical activity. Mouse CSF-1 does not interact with the human CSF-1 receptor but was active on pig. By contrast, porcine CSF-1 was active on mouse, human, cat and dog cells. IL-34 was previously shown to be species-specific, with mouse and human proteins demonstrating limited cross-species activity. The pig CSF-1R was equally responsive to both mouse and human IL-34. Based upon the published crystal structures of CSF-1/CSF-1R and IL34/CSF-1R complexes, we discuss the molecular basis for the species specificity.

Platelet-derived growth factors and their receptors: structural and functional perspectives

The four types of platelet-derived growth factors (PDGFs) and the two types of PDGF receptors (PDGFRs, which belong to class III receptor tyrosine kinases) have important functions in the development of connective tissue cells. Recent structural studies have revealed novel mechanisms of PDGFs in propeptide loading and receptor recognition/activation. The detailed structural understanding of PDGF-PDGFR signaling has provided a template that can aid therapeutic intervention to counteract the aberrant signaling of this normally silent pathway, especially in proliferative diseases such as cancer. This review summarizes the advances in the PDGF system with a focus on relating the structural and functional understandings, and discusses the basic aspects of PDGFs and PDGFRs, the mechanisms of activation, and the insights into the therapeutic antagonism of PDGFRs. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Extracellular assembly and activation principles of oncogenic class III receptor tyrosine kinases

Intracellular signalling cascades initiated by class III receptor tyrosine kinases (RTK-IIIs) and their cytokine ligands contribute to haematopoiesis and mesenchymal tissue development. They are also implicated in a wide range of inflammatory disorders and cancers. Recent snapshots of RTK-III ectodomains in complex with cognate cytokines have revealed timely insights into the structural determinants of RTK-III activation, evolution and pathology. Importantly, candidate 'driver' and 'passenger' mutations that have been identified in RTK-IIIs can now be collectively mapped for the first time to structural scaffolds of the corresponding RTK-III ectodomains. Such insights will generate a renewed interest in dissecting the mechanistic effects of such mutations and their therapeutic relevance.

Allosteric competitive inactivation of hematopoietic CSF-1 signaling by the viral decoy receptor BARF1

Hematopoietic human colony-stimulating factor 1 (hCSF-1) is essential for innate and adaptive immunity against viral and microbial infections and cancer. The human pathogen Epstein-Barr virus secretes the lytic-cycle protein BARF1 that neutralizes hCSF-1 to achieve immunomodulation. Here we show that BARF1 binds the dimer interface of hCSF-1 with picomolar affinity, away from the cognate receptor-binding site, to establish a long-lived complex featuring three hCSF-1 at the periphery of the BARF1 toroid. BARF1 locks dimeric hCSF-1 into an inactive conformation, rendering it unable to signal via its cognate receptor on human monocytes. This reveals a new functional role for hCSF-1 cooperativity in signaling. We propose a new viral strategy paradigm featuring an allosteric decoy receptor of the competitive type, which couples efficient sequestration and inactivation of the host growth factor to abrogate cooperative assembly of the cognate signaling complex.

The yin-yang nature of CSF1R-binding cytokines

Most myeloid cells express the growth-factor receptor CSF1R. Recognition of interleukin 34 by CSF1R is required for the development of tissue-resident Langerhans cells and microglia, which explains the independence of their growth from CSF1.

Plenary perspective: the complexity of constitutive and inducible gene expression in mononuclear phagocytes

Monocytes and macrophages differentiate from progenitor cells under the influence of colony-stimulating factors. Genome-scale data have enabled the identification of the set of genes that distinguishes macrophages from other cell types and the ways in which thousands of genes are regulated in response to pathogen challenge. Although there has been a focus on a small subset of lineage-enriched transcription factors, such as PU.1, more than one-half of the transcription factors in the genome can be expressed in macrophage lineage cells under some state of activation, and they interact in a complex network. The network architecture is conserved across species, but many of the target genes evolve rapidly and differ between mouse and human. The data and publication deluge related to macrophage biology require the development of new analytical tools and ways of presenting information in an accessible form.

IL-34 is a tissue-restricted ligand of CSF1R required for the development of Langerhans cells and microglia

The differentiation of bone marrow-derived progenitor cells into monocytes, tissue macrophages and some dendritic cell (DC) subtypes requires the growth factor CSF1 and its receptor, CSF1R. Langerhans cells (LCs) and microglia develop from embryonic myeloid precursor cells that populate the epidermis and central nervous system (CNS) before birth. Notably, LCs and microglia are present in CSF1-deficient mice but absent from CSF1R-deficient mice. Here we investigated whether an alternative CSF1R ligand, interleukin 34 (IL-34), is responsible for this discrepancy. Through the use of IL-34-deficient (Il34(LacZ/LacZ)) reporter mice, we found that keratinocytes and neurons were the main sources of IL-34. Il34(LacZ/LacZ) mice selectively lacked LCs and microglia and responded poorly to skin antigens and viral infection of the CNS. Thus, IL-34 specifically directs the differentiation of myeloid cells in the skin epidermis and CNS.

The mechanism of shared but distinct CSF-1R signaling by the non-homologous cytokines IL-34 and CSF-1

Interleukin-34 (IL-34) and colony stimulating factor-1 (CSF-1) both signal through the CSF-1R receptor tyrosine kinase, but they have no sequence homology, and their functions and signaling activities are not identical. We report the crystal structures of mouse IL-34 alone and in complex with the N-terminal three immunoglobulin-like domains (D1-D3) of mouse CSF-1R. IL-34 is structurally related to other helical hematopoietic cytokines, but contains two additional helices integrally associated with the four shared helices. The non-covalently linked IL-34 homodimer recruits two copies of CSF-1R on the sides of the helical bundles, with an overall shape similar to the CSF-1:CSF-1R complex, but the flexible linker between CSF-1R D2 and D3 allows these domains to clamp IL-34 and CSF-1 at different angles. Functional dissection of the IL-34:CSF-1R interface indicates that the hydrophobic interactions, rather than the salt bridge network, dominate the biological activity of IL-34. To degenerately recognize two ligands with completely different surfaces, CSF-1R apparently takes advantage of different subsets of a chemically inert surface that can be tuned to fit different ligand shapes. Differentiated signaling between IL-34 and CSF-1 is likely achieved by the relative thermodynamic independence of IL-34 vs. negative cooperativity of CSF-1 at the receptor-recognition sites, in combination with the difference in hydrophobicity which dictates a more stable IL-34:CSF-1R complex compared to the CSF-1:CSF-1R complex.