Coating M-CSF on plastic surface results in the generation of increased numbers of macrophages in vitro.

Macrophages are one of the important cell types in the innate immune system that are present in various anatomical regions of the body and promote early control of pathogens. The relative proportion of macrophages in various lymphoid and non-lymphoid regions is small, and as such it is tedious to purify these cells to homogeneity. Culture of bone marrow precursors with macrophage colony-stimulating factor (M-CSF) results in their differentiation to macrophages, however this procedure results in low numbers of differentiated macrophages. Herein we reveal a new approach of generating increased numbers of differentiated macrophages from bone marrow precursors. We show that M-CSF delivered in a plate-bound form results in the differentiation of significantly more macrophages in comparison to soluble M-CSF. Furthermore, the macrophages differentiated with plate-bound M-CSF display increased metabolic activity and cell death following infection with pathogens.

Intratumoral delivery of M-CSF by calcium crosslinked polymer micelles enhances cancer immunotherapy.

Immunotherapy has shown promising results in multiple malignancies. However, there are still significant challenges in cancer immunotherapy including the powerful immunosuppressive tumor microenvironment and adverse off-target side effects. Nanomaterials with defined physico-biochemical properties are versatile drug delivery platforms that may address these key technical challenges faced by cancer immunotherapy. Here, a tumor acidity-responsive biomacromolecule delivery system was designed to intratumorally deliver an immune-activating cytokine, macrophage colony-stimulating factor (M-CSF) and attenuate the acidic microenvironment. This nanoparticle was prepared by introducing CaCO3 as a crosslinker to form an M-CSF-loaded stable micelle (NP/M-CSF/CaCO3). Administration of NP/M-CSF/CaCO3 significantly inhibited tumor growth by enhancing T cell-mediated anti-tumor immune responses and reversing the TAM-mediated immunosuppression. This study provides new avenues for cascade amplification of the antitumor effects by targeting the tumor microenvironment. This approach may also help avoid unwanted complications.

Optimization of Serum-Free Culture Conditions for Propagation of Putative Buffalo (Bubalus bubalis) Spermatogonial Stem Cells.

Spermatogonial stem cells (SSCs) self-renew and produce a large number of differentiated germ cells to maintain normal spermatogenesis. However, the growth factors crucial for SSC self-renewal and the mechanism underlying this process remain unclear. In the present study, a serum-free culture media was used to evaluate the effect of several growth factors on the expression of some SSC markers and self-renewal related genes. The putative SSCs were cultured on buffalo Sertoli cell feeder layer in KO-DMEM +10% KOSR. The colony formation was observed between 7 and 10 days. The putative SSC colonies also expressed markers specific for undifferentiated type A spermatogonia and pluripotency markers. After 15 days, relative mRNA expression study revealed that 20 ng/mL concentration of Glial cell line-derived neurotrophic factor (GDNF) upregulated the expression of PLZF, TAF4B, and THY1. Furthermore, supplementation of a combination of 20 ng/mL GDNF, 10 ng/mL basic fibroblast growth factor (bFGF), 1000 IU/mL leukemia inhibitory factor (LIF), and 1 ng/mL colony stimulating factor 1 (CSF1) upregulated the expression of PLZF, TAF4B, BCL6B, and ID4 genes. These results demonstrated that our defined culture media in combination with GDNF, bFGF, LIF, and CSF1 well supported SSC self-renewal.

Identification of Novel Natural Substrates of Fibroblast Activation Protein-alpha by Differential Degradomics and Proteomics.

Fibroblast activation protein-alpha (FAP) is a cell-surface transmembrane-anchored dimeric protease. This unique, constitutively active serine protease has both dipeptidyl aminopeptidase and endopeptidase activities and can hydrolyze the post-proline bond. FAP expression is very low in adult organs but is upregulated by activated fibroblasts in sites of tissue remodeling, including fibrosis, atherosclerosis, arthritis and tumors. To identify the endogenous substrates of FAP, we immortalized primary mouse embryonic fibroblasts (MEFs) from FAP gene knockout embryos and then stably transduced them to express either enzymatically active or inactive FAP. The MEF secretomes were then analyzed using degradomic and proteomic techniques. Terminal amine isotopic labeling of substrates (TAILS)-based degradomics identified cleavage sites in collagens, many other extracellular matrix (ECM) and associated proteins, and lysyl oxidase-like-1, CXCL-5, CSF-1, and C1qT6, that were confirmed in vitro In addition, differential metabolic labeling coupled with quantitative proteomic analysis also implicated FAP in ECM-cell interactions, as well as with coagulation, metabolism and wound healing associated proteins. Plasma from FAP-deficient mice exhibited slower than wild-type clotting times. This study provides a significant expansion of the substrate repertoire of FAP and provides insight into the physiological and potential pathological roles of this enigmatic protease.

Differentiation of Human-Induced Pluripotent Stem Cells to Macrophages for Disease Modeling and Functional Genomics.

Macrophages play important roles in many diseases. We describe a protocol and the associated resources for the differentiation of human induced pluripotent stem cell-derived macrophages (IPSDM) and their applications in understanding human macrophage physiology and relevant diseases. The protocol uses an embryoid body-based approach with a combination of serum-free condition for hematopoiesis specification, followed by adherent culture with serum and M-CSF for myeloid expansion and macrophage maturation. The protocol produced an almost pure culture of CD45+ /CD18+ macrophages yielding up to 2 × 107 cells per 6-well plate of iPSCs within 24 days, demonstrating high efficiency, purity, and scalability. The IPSDM and monocyte-derived macrophages (HMDM) cultured in the same medium were compared at morphological, functional and transcriptomic levels by RNA-sequencing. IPSDM and HMDM showed broadly similar profiles of coding transcriptome, alternative splicing events, and long noncoding RNAs, with advantages and successful applications in disease modeling using patients-derived and CRISPR-edited iPSC lines. © 2018 by John Wiley & Sons, Inc.

A dual-specific macrophage colony-stimulating factor antagonist of c-FMS and αvß3 integrin for osteoporosis therapy.

There is currently a demand for new highly efficient and specific drugs to treat osteoporosis, a chronic bone disease affecting millions of people worldwide. We have developed a combinatorial strategy for engineering bispecific inhibitors that simultaneously target the unique combination of c-FMS and αvß3 integrin, which act in concert to facilitate bone resorption by osteoclasts. Using functional fluorescence-activated cell sorting (FACS)-based screening assays of random mutagenesis macrophage colony-stimulating factor (M-CSF) libraries against c-FMS and αvß3 integrin, we engineered dual-specific M-CSF mutants with high affinity to both receptors. These bispecific mutants act as functional antagonists of c-FMS and αvß3 integrin activation and hence of osteoclast differentiation in vitro and osteoclast activity in vivo. This study thus introduces a versatile platform for the creation of new-generation therapeutics with high efficacy and specificity for osteoporosis and other bone diseases. It also provides new tools for studying molecular mechanisms and the cell signaling pathways that mediate osteoclast differentiation and function.

A designer self-assembled supramolecule amplifies macrophage immune responses against aggressive cancer.

Effectively activating macrophages that can 'eat' cancer cells is challenging. In particular, cancer cells secrete macrophage colony stimulating factor (MCSF), which polarizes tumour-associated macrophages from an antitumour M1 phenotype to a pro-tumourigenic M2 phenotype. Also, cancer cells can express CD47, an 'eat me not' signal that ligates with the signal regulatory protein alpha (SIRPα) receptor on macrophages to prevent phagocytosis. Here, we show that a supramolecular assembly consisting of amphiphiles inhibiting the colony stimulating factor 1 receptor (CSF-1R) and displaying SIRPα-blocking antibodies with a drug-to-antibody ratio of 17,000 can disable both mechanisms. The supramolecule homes onto SIRPα on macrophages, blocking the CD47-SIRPα signalling axis while sustainedly inhibiting CSF-1R. The supramolecule enhances the M2-to-M1 repolarization within the tumour microenvironment, and significantly improves antitumour and antimetastatic efficacies in two aggressive animal models of melanoma and breast cancer, with respect to clinically available small-molecule and biologic inhibitors of CSF-1R signalling. Simultaneously blocking the CD47-SIRPα and MCSF-CSF-1R signalling axes may constitute a promising immunotherapy.

AMP-activated protein kinase α1 promotes atherogenesis by increasing monocyte-to-macrophage differentiation.

Monocyte-to-macrophage differentiation, which can be initiated by physiological or atherogenic factors, is a pivotal process in atherogenesis, a disorder in which monocytes adhere to endothelial cells and subsequently migrate into the subendothelial spaces, where they differentiate into macrophages and macrophage-derived foam cells and cause atherosclerotic lesions. However, the monocyte-differentiation signaling pathways that are activated by atherogenic factors are poorly defined. Here we report that the AMP-activated protein kinase α1 (AMPKα1) in monocytes promotes atherosclerosis by increasing monocyte differentiation and survival. Exposure of monocytes to oxidized low-density lipoprotein, 7-ketocholesterol, phorbol 12-myristate 13-acetate, or macrophage colony-stimulated factor (M-CSF) significantly activated AMPK and promoted monocyte-to-macrophage differentiation. M-CSF-activated AMPK is via M-CSF receptor-dependent reactive oxygen species production. Consistently, genetic deletion of AMPKα1 or pharmacological inhibition of AMPK blunted monocyte-to-macrophage differentiation and promoted monocyte/macrophage apoptosis. Compared with apolipoprotein E knock-out (ApoE-/-) mice, which show impaired clearing of plasma lipoproteins and spontaneously develop atherosclerosis, ApoE-/-/AMPKα1-/- mice showed reduced sizes of atherosclerotic lesions and lesser numbers of macrophages in the lesions. Furthermore, aortic lesions were decreased in ApoE-/- mice transplanted with ApoE-/-/AMPKα1-/- bone marrow and in myeloid-specific AMPKα1-deficient ApoE-/- mice. Finally, rapamycin treatment, which abolished delayed monocyte differentiation in ApoE-/-/AMPKα1-/- mice, lost its atherosclerosis-lowering effects in these mice. Mechanistically, we found that AMPKα1 regulates FoxO3-dependent expression of both LC3 and ULK1, which are two important autophagy-related markers. Rapamycin treatment increased FoxO3 activity as well as LC3 and ULK1 expressions in macrophages from AMPKα1-/- mice. Our results reveal that AMPKα1 deficiency impairs autophagy-mediated monocyte differentiation and decreases monocyte/macrophage survival, which attenuates atherosclerosis in ApoE-/- mice in vivo.

Effects of different orthodontic retention protocols on the periodontal health of mandibular incisors.

OBJECTIVES: To test the following two hypotheses: 1) different types of retainers result in distinct levels of biomarkers in gingival crevicular fluid (GCF) and 2) the retainer bonded to all mandibular anterior teeth induces more detrimental outcomes to the periodontium. SETTING AND SAMPLE POPULATION: The Department of Orthodontics at the University of Florida. The population consisted of individuals in the retention phase of orthodontic treatment. MATERIAL AND METHODS: This was a cross-sectional study that enrolled 36 individuals. Subjects in group 1 had retainers bonded to the mandibular canines only. Group 2 consisted of individuals having retainers bonded to all mandibular anterior teeth. Group 3 included patients using mandibular removable retainers. After clinical examination, GCF was collected from the mandibular incisor and biomarker levels were compared between the groups. RESULTS: Plaque accumulation and gingivitis differed significantly among groups, with the highest median values in group 2 subjects. Pairwise comparison of the groups with respect to gingivitis showed significant differences between groups 1 and 2. Significant differences among groups were detected for RANKL, OPG, OPN, M-CSF, MMP-3, and MMP-9. The ratio RANKL/OPG was significantly higher in group 2 subjects, with pairwise comparisons indicating that groups 1 and 2 differed from group 3. CONCLUSION: An association was found between orthodontic retention groups and GCF biomarker levels, which should be further explored in longitudinal studies. The presence of retainers bonded to all anterior teeth seems to increase plaque accumulation and gingivitis.

Inflammatory response of TLR4 deficient spleen macrophages (CRL 2471) to Brucella abortus S19 and an isogenic ΔmglA deletion mutant.

UNLABELLED: Brucellosis is a worldwide distributed zoonosis caused by members of the genus Brucella. One of them, Brucella abortus, is the etiological agent of bovine brucellosis. With the attenuated strain B. abortus S19 a vaccine is available. However, both, virulence (safety) and the ability to induce a protective B and T cell response (efficacy) have to be tested in suitable assays before successful use in the field. For this purpose, several macrophage cell lines of various origins have been used while splenic macrophages are the preferred host cells in vivo. We here characterized the in vitro response of the murine splenic macrophage cell line CRL 2471(I-13.35) to B. abortus. This cell line still depends on the presence of colony-stimulating factor 1 (CSF1) and is derived from LPS resistant (TLR4 deficient) C3H/HeJ mice. For infection the vaccine strain B. abortus S19A as well as the formerly described isogenic deletion mutant B. abortus S19A ΔmglA 3.14 were used. While numbers of viable bacteria did not differ significantly between the vaccine strain and the deletion mutant at 6h post infection, a higher bacterial load was measured in case of the mutant at 24h and 48h after infection. This was also true, when IFNγ was used for macrophage activation. A comprehensive gene expression profile of macrophages was analysed 6 and 24h after infection by means of an RT-PCR based gene expression array. The mutant strain B. abortus S19A ΔmglA 3.14 elicited a stronger cellular response of the splenic macrophages as compared to the parental vaccine strain. This was most prominent for the pro-inflammatory cytokines IL-1α, IL-1ß, TNF-α and IL6 as well as for the chemokine ligands CXCL1, CXCL2, CXCL10, CCL2, CCL5, CCL7, CCL17 and the co-stimulatory molecules CD40 and ICAM1. While these differences were also present in IFNγ-stimulated macrophages, an addition of IFNγ after infection not only resulted in a dramatic increase of the translation of the afore mentioned genes but also resulted in the translation of IFNß1, IL12ß, MIP1α and ß (CCL3, CCL4), NOS2 (and SOD2) and FAS. CONCLUSION: The TLR4 deficient murine splenic macrophage cell line CRL 2471 was used for the first time for the characterization of macrophage-Brucella interaction to investigate the pre-immune phase of brucellosis in vitro. Typical pro-inflammatory cytokines and certain surface receptors were differentially induced by B. abortus S19 A and an isogenic ΔmglA deletion mutant in vitro. This model may be useful for further studies to characterize the inflammatory response of splenic macrophages to intracellular gram-negative bacteria avoiding cell responses to soluble LPS.

Combinatorial and Computational Approaches to Identify Interactions of Macrophage Colony-stimulating Factor (M-CSF) and Its Receptor c-FMS.

The molecular interactions between macrophage colony-stimulating factor (M-CSF) and the tyrosine kinase receptor c-FMS play a key role in the immune response, bone metabolism, and the development of some cancers. Because no x-ray structure is available for the human M-CSF · c-FMS complex, the binding epitope for this complex is largely unknown. Our goal was to identify the residues that are essential for binding of the human M-CSF to c-FMS. For this purpose, we used a yeast surface display (YSD) approach. We expressed a combinatorial library of monomeric M-CSF (M-CSFM) single mutants and screened this library to isolate variants with reduced affinity for c-FMS using FACS. Sequencing yielded a number of single M-CSFM variants with mutations both in the direct binding interface and distant from the binding site. In addition, we used computational modeling to map the identified mutations onto the M-CSFM structure and to classify the mutations into three groups as follows: those that significantly decrease protein stability; those that destroy favorable intermolecular interactions; and those that decrease affinity through allosteric effects. To validate the YSD and computational data, M-CSFM and three variants were produced as soluble proteins; their affinity and structure were analyzed; and very good correlations with both YSD data and computational predictions were obtained. By identifying the M-CSFM residues critical for M-CSF · c-FMS interactions, we have laid down the basis for a deeper understanding of the M-CSF · c-FMS signaling mechanism and for the development of target-specific therapeutic agents with the ability to sterically occlude the M-CSF·c-FMS binding interface.

Structure and Assembly Mechanism of the Signaling Complex Mediated by Human CSF-1.

Human colony-stimulating factor 1 receptor (hCSF-1R) is unique among the hematopoietic receptors because it is activated by two distinct cytokines, CSF-1 and interleukin-34 (IL-34). Despite ever-growing insights into the central role of hCSF-1R signaling in innate and adaptive immunity, inflammatory diseases, and cancer, the structural basis of the functional dichotomy of hCSF-1R has remained elusive. Here, we report crystal structures of ternary complexes between hCSF-1 and hCSF-1R, including their complete extracellular assembly, and propose a mechanism for the cooperative human CSF-1:CSF-1R complex that relies on the adoption by dimeric hCSF-1 of an active conformational state and homotypic receptor interactions. Furthermore, we trace the cytokine-binding duality of hCSF-1R to a limited set of conserved interactions mediated by functionally equivalent residues on CSF-1 and IL-34 that play into the geometric requirements of hCSF-1R activation, and map the possible mechanistic consequences of somatic mutations in hCSF-1R associated with cancer.

IL-34 and M-CSF form a novel heteromeric cytokine and regulate the M-CSF receptor activation and localization.

Interleukin-34 (IL-34) is a newly-discovered homodimeric cytokine that regulates, like Macrophage Colony-Stimulating Factor (M-CSF), the differentiation of the myeloid lineage through M-CSF receptor (M-CSFR) signaling pathways. To date, both cytokines have been considered as competitive cytokines with regard to the M-CSFR. The aim of the present work was to study the functional relationships of these cytokines on cells expressing the M-CSFR. We demonstrate that simultaneous addition of M-CSF and IL-34 led to a specific activation pattern on the M-CSFR, with higher phosphorylation of the tyrosine residues at low concentrations. Similarly, both cytokines showed an additive effect on cellular proliferation or viability. In addition, BIAcore experiments demonstrated that M-CSF binds to IL-34, and molecular docking studies predicted the formation of a heteromeric M-CSF/IL-34 cytokine. A proximity ligation assay confirmed this interaction between the cytokines. Finally, co-expression of the M-CSFR and its ligands differentially regulated M-CSFR trafficking into the cell. This study establishes a new foundation for the understanding of the functional relationship between IL-34 and M-CSF, and gives a new vision for the development of therapeutic approaches targeting the IL-34/M-CSF/M-CSFR axis.

Grouper (Epinephelus coioides) IL-34/MCSF2 and MCSFR1/MCSFR2 were involved in mononuclear phagocytes activation against Cryptocaryon irritans infection.

MCSF and its well-known receptor MCSFR had been well studied in humans, regulating the differentiation, proliferation, and survival of the mononuclear phagocyte system. IL-34, which is an alternative ligand of MCSF receptor, was recently identified as a novel cytokine and functionally overlaps with MCSF. However, the functional study of these receptors and their ligands in fish are largely unknown. In the present study, the cDNA of two potential grouper MCSFR ligands have been cloned, EcIL-34 (657 bp) and EcMCSF2 (804 bp), as well as an additional copy of grouper MCSFR, EcMCSFR2 (3141 bp). Sequence analysis showed that these three molecules had higher identities with other fish counterparts compared to mammals and their conserved structures and important functional residues were also analyzed. Tissue distribution analysis showed that EcIL-34 is dominant in brain, gill and spleen compared to EcMCSF2, which is dominant in head kidney, trunk kidney, skin, heart and muscle. EcMCSFR1 was dominant in the most tissues except head kidney and liver compared to EcMCSFR2. The different tissue distribution patterns of these two grouper MCSF receptors and their two ligands indicate the different mononuclear phagocyte differentiation and activation modes in different tissues. In Cryptocaryon irritans infected grouper, EcIL-34 and EcMCSFR2 were the most strongly up-regulated ligand and receptor in the infected sites, gill and skin. Their up-regulation confirmed the proliferation and activation of phagocytes in C. irritans infected sites, which would improve the antigen presentation and elicit the host local specific immune response. In C. irritans infected grouper head kidney, both ligands EcIL-34 and EcMCSF2 (especially EcMCSF2) were up-regulated, but both receptors EcMCSFR1 and EcMCSFR2 were down-regulated, which indicated that the phagocytes differentiation and proliferation may have occurred in this hemopoietic organ, and after that they migrated to the infected cites. The down-regulation of EcIL-34 and EcMCSF2 and no significant change of EcMCSFR1 and EcMCSFR2 in most time point of grouper spleen showed it was less involved in phagocytes response to C. irritans infection.

Cloning and expression analysis of grouper (Epinephelus coioides) M-CSFR gene post Cryptocaryon irritans infection and distribution of M-CSFR(+) cells.

The M-CSF/M-CSFR system plays a central role in the cell survival, proliferation, differentiation and maturation of the monocyte/macrophage lineage. In present study, we cloned the sequence of the M-CSFR cDNA from the orange-spotted grouper (Epinephelus coioides). Sequence analysis reveals that ten cysteines in the extracellular immunoglobulin-like (Ig-like) domains of EcM-CSFR are conserved in fish and mammals, its nine possible N-glycosylation sites are conserved in fish but not mammals, 7 of 8 identified mammal M-CSFR intracellular autophosphorylation tyrosine sites was found in EcM-CSFR. Real-time PCR showed that the constitutive expression level of EcM-CSFR was the highest in the spleen, less in the gill, kidney, head kidney and liver, least in the blood, skin, gut and thymus. A rabbit anti-EcM-CSFR polyclonal antibody against the recombinant EcM-CSFR extracellular domain was developed and it was efficient in labeling the monocytes and macrophages isolated from the head kidney. Immunochemistry analysis showed that M-CSFR(+) cells located in all tested paraffin-embedded tissues and M-CSFR(+) cell centres with the characteristic of melano-macrophage centres(MMCs) was found in the spleen, head kidney, kidney, gut and liver. All these results indicate the widespread distribution of macrophages in grouper tissues and its importance in fish immune system. In Crytocaryon irritans infected grouper, EcM-CSFR was transient up-regulated and rapidly down-regulated in skin, gill, head kidney and spleen. The possible activation mechanism of macrophage via EcM-CSFR signal transduction in the fish anti-C. irritans infection was discussed.

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.

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.

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.

Multipronged attenuation of macrophage-colony stimulating factor signaling by Epstein-Barr virus BARF1.

The ubiquitous EBV causes infectious mononucleosis and is associated with several types of cancers. The EBV genome encodes an early gene product, BARF1, which contributes to pathogenesis, potentially through growth-altering and immune-modulating activities, but the mechanisms for such activities are poorly understood. We have determined the crystal structure of BARF1 in complex with human macrophage-colony stimulating factor (M-CSF), a hematopoietic cytokine with pleiotropic functions in development and immune response. BARF1 and M-CSF form a high-affinity, stable, ring-like complex in both solution and the crystal, with a BARF1 hexameric ring surrounded by three M-CSF dimers in triangular array. The binding of BARF1 to M-CSF dramatically reduces but does not completely abolish M-CSF binding and signaling through its cognate receptor FMS. A three-pronged down-regulation mechanism is proposed to explain the biological effect of BARF1 on M-CSF:FMS signaling. These prongs entail control of the circulating and effective local M-CSF concentration, perturbation of the receptor-binding surface of M-CSF, and imposition of an unfavorable global orientation of the M-CSF dimer. Each prong may reduce M-CSF:FMS signaling to a limited extent but in combination may alter M-CSF:FMS signaling dramatically. The downregulating mechanism of BARF1 underlines a viral modulation strategy, and provides a basis for understanding EBV pathogenesis.

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.