Cloning of the low-affinity murine granulocyte-macrophage colony-stimulating factor receptor and reconstitution of a high-affinity receptor complex

GM-CSF, IL-3, and IL-5 Family of Cytokines: Regulators of Inflammation

The β common chain cytokines GM-CSF, IL-3, and IL-5 regulate varied inflammatory responses that promote the rapid clearance of pathogens but also contribute to pathology in chronic inflammation. Therapeutic interventions manipulating these cytokines are approved for use in some cancers as well as allergic and autoimmune disease, and others show promising early clinical activity. These approaches are based on our understanding of the inflammatory roles of these cytokines; however, GM-CSF also participates in the resolution of inflammation, and IL-3 and IL-5 may also have such properties. Here, we review the functions of the β common cytokines in health and disease. We discuss preclinical and clinical data, highlighting the potential inherent in targeting these cytokine pathways, the limitations, and the important gaps in understanding of the basic biology of this cytokine family.

Dual Role of GM-CSF as a Pro-Inflammatory and a Regulatory Cytokine: Implications for Immune Therapy

Granulocyte macrophage colony stimulating factor (GM-CSF) is generally recognized as an inflammatory cytokine. Its inflammatory activity is primarily due its role as a growth and differentiation factor for granulocyte and macrophage populations. In this capacity, among other clinical applications, it has been used to bolster anti-tumor immune responses. GM-CSF-mediated inflammation has also been implicated in certain types of autoimmune diseases, including rheumatoid arthritis and multiple sclerosis. Thus, agents that can block GM-CSF or its receptor have been used as anti-inflammatory therapies. However, a review of literature reveals that in many situations GM-CSF can act as an anti-inflammatory/regulatory cytokine. We and others have shown that GM-CSF can modulate dendritic cell differentiation to render them "tolerogenic," which, in turn, can increase regulatory T-cell numbers and function. Therefore, the pro-inflammatory and regulatory effects of GM-CSF appear to depend on the dose and the presence of other relevant cytokines in the context of an immune response. A thorough understanding of the various immunomodulatory effects of GM-CSF will facilitate more appropriate use and thus further enhance its clinical utility.

Molecular cloning, sequencing and structural studies of granulocyte-macrophage colony-stimulating factor (GM-CSF) from Indian water buffalo (Bubalus bubalis)

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine that is essential for growth and development of progenitors of granulocytes and monocytes/macrophages. In this study, we report molecular cloning, sequencing and characterization of GM-CSF from Indian water buffalo, Bubalus bubalis. In addition, we performed sequence and structural analysis for buffalo GM-CSF. Buffalo GM-CSF has been compared with 17 mammalian GM-CSFs using multiple sequence alignment and phylogenetic tree. Three-dimensional model for buffalo GM-CSF and human receptor complex was built using homology modelling to study cross-reactivity between two species. Detailed analysis was performed to study GM-CSF interface and various interactions at the interface.

CBL linker region and RING finger mutations lead to enhanced granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling via elevated levels of JAK2 and LYN

Juvenile myelomonocytic leukemia (JMML) is characterized by hypersensitivity to granulocyte-macrophage colony-stimulating factor (GM-CSF). SHP2, NF-1, KRAS, and NRAS are mutated in JMML patients, leading to aberrant regulation of RAS signaling. A subset of JMML patients harbor CBL mutations associated with 11q acquired uniparental disomy. Many of these mutations are in the linker region and the RING finger of CBL, leading to a loss of E3 ligase activity. We investigated the mechanism by which CBL-Y371H, a linker region mutant, and CBL-C384R, a RING finger mutant, lead to enhanced GM-CSF signaling. Expression of CBL mutants in the TF-1 cell line resulted in enhanced survival in the absence of GM-CSF. Cells expressing CBL mutations displayed increased phosphorylation of GM-CSF receptor βc subunit in response to stimulation, although expression of total GM-CSFR βc was lower. This suggested enhanced kinase activity downstream of GM-CSFR. JAK2 and LYN kinase expression is elevated in CBL-Y371H and CBL-C384R mutant cells, resulting in enhanced phosphorylation of CBL and S6 in response to GM-CSF stimulation. Incubation with the JAK2 inhibitor, TG101348, abolished the increased phosphorylation of GM-CSFR βc in cells expressing CBL mutants, whereas treatment with the SRC kinase inhibitor dasatinib resulted in equalization of GM-CSFR βc phosphorylation signal between wild type CBL and CBL mutant samples. Dasatinib treatment inhibited the elevated phosphorylation of CBL-Y371H and CBL-C384R mutants. Our study indicates that CBL linker and RING finger mutants lead to enhanced GM-CSF signaling due to elevated kinase expression, which can be blocked using small molecule inhibitors targeting specific downstream pathways.

The GM-CSF/IL-3/IL-5 cytokine receptor family: from ligand recognition to initiation of signaling

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 are members of a discrete family of cytokines that regulates the growth, differentiation, migration and effector function activities of many hematopoietic cells and immunocytes. These cytokines are involved in normal responses to infectious agents, bridging innate and adaptive immunity. However, in certain cases, the overexpression of these cytokines or their receptors can lead to excessive or aberrant initiation of signaling resulting in pathological conditions, with chronic inflammatory diseases and myeloid leukemias the most notable examples. Recent crystal structures of the GM-CSF receptor ternary complex and the IL-5 binary complex have revealed new paradigms of cytokine receptor activation. Together with a wealth of associated structure-function studies, they have significantly enhanced our understanding of how these receptors recognize cytokines and initiate signals across cell membranes. Importantly, these structures provide opportunities for structure-based approaches for the discovery of novel and disease-specific therapeutics. In addition, recent biochemical evidence has suggested that the GM-CSF/IL-3/IL-5 receptor family is capable of interacting productively with other membrane proteins at the cell surface. Such interactions may afford additional or unique biological activities and might be harnessed for selective modulation of the function of these receptors in disease.

The SWI/SNF protein ATRX co-regulates pseudoautosomal genes that have translocated to autosomes in the mouse genome

BACKGROUND

Pseudoautosomal regions (PAR1 and PAR2) in eutherians retain homologous regions between the X and Y chromosomes that play a critical role in the obligatory X-Y crossover during male meiosis. Genes that reside in the PAR1 are exceptional in that they are rich in repetitive sequences and undergo a very high rate of recombination. Remarkably, murine PAR1 homologs have translocated to various autosomes, reflecting the complex recombination history during the evolution of the mammalian X chromosome.

RESULTS

We now report that the SNF2-type chromatin remodeling protein ATRX controls the expression of eutherian ancestral PAR1 genes that have translocated to autosomes in the mouse. In addition, we have identified two potentially novel mouse PAR1 orthologs.

CONCLUSION

We propose that the ancestral PAR1 genes share a common epigenetic environment that allows ATRX to control their expression.

Cross-talk between ICAM-1 and granulocyte-macrophage colony-stimulating factor receptor signaling modulates eosinophil survival and activation

Reversal of eosinophilic inflammation has been an elusive therapeutic goal in the management of asthma pathogenesis. In this regard, GM-CSF is a primary candidate cytokine regulating eosinophil activation and survival in the lung; however, its molecular mechanism of propagation and maintenance of stimulated eosinophil activation is not well understood. In this study, we elucidate those late interactions occurring between the GM-CSF receptor and activated eosinophil signaling molecules. Using coimmunoprecipitation with GM-CSF-stimulated eosinophils, we have identified that the GM-CSF receptor beta-chain (GMRbeta) interacted with ICAM-1 and Shp2 phosphatase, as well as Slp76 and ADAP adaptor proteins. Separate experiments using affinity binding with a tyrosine-phosphorylated peptide containing an ITIM (ICAM-1 residues 480-488) showed binding to Shp2 phosphatase and GMRbeta. However, the interaction of GMRbeta with the phosphorylated ICAM-1-derived peptide was observed only with stimulated eosinophil lysates, suggesting that the interaction of GMRbeta with ICAM-1 required phosphorylated Shp2 and/or phosphorylated GMRbeta. Importantly, we found that inhibition of ICAM-1 in activated eosinophils blocked GM-CSF-induced expression of c-fos, c-myc, IL-8, and TNF-alpha. Moreover, inhibition of ICAM-1 expression with either antisense oligonucleotide or an ICAM-1-blocking Ab effectively inhibited ERK activation and eosinophil survival. We concluded that the interaction between ICAM-1 and the GM-CSF receptor was essential for GM-CSF-induced eosinophil activation and survival. Taken together, these results provide novel mechanistic insights defining the interaction between ICAM-1 and the GM-CSF receptor and highlight the importance of targeting ICAM-1 and GM-CSF/IL-5/IL-3 receptor systems as a therapeutic strategy to counter eosinophilia in asthma.

Characterisation of the expression and function of the GM-CSF receptor alpha-chain in mice

The granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hematopoietic cytokine able to regulate a variety of cell functions including differentiation of macrophages and granulocytes, dendritic cell development and the maintenance of homeostasis. It binds specifically to its receptor, which is composed of a cytokine-specific α-chain (GM-CSF receptor α-chain, GMRα) and a β-chain shared with the receptors for interleukin-3 and interleukin-5. In this report, we present a comprehensive study of GMRα in the mouse. We have found that the mouse GMRα is polymorphic and alternatively spliced. In the absence of specific antibodies, we generated a novel chimeric protein containing the Fc fragment of human IgG1 coupled to mouse GM-CSF, which was able to specifically bind to GMRα and induce proliferation of GMRα-transduced Ba/F3 cells. We used this reagent to perform the first detailed FACS study of the surface expression of mouse GMRα by leucocytes. Highest expression was found on monocytes and granulocytes, and variable expression on tissue macrophages. The GM-CSF receptor in mice is specifically expressed by myeloid cells and is useful for the detection of novel uncharacterised myeloid populations. The ability to detect GM-CSF receptor expression in experimental studies should greatly facilitate the analysis of its role in immune pathologies.

Effect of granulocyte-macrophage colony stimulating factor on growth, resistance to freezing and thawing and re-expansion of murine blastocysts

Two-cell murine embryos were cultured for 72 h in the presence or absence of granulocyte-macrophage colony stimulating factor (GM-CSF), frozen for 60 days and, after thawing, cultured for an additional 24 h in the presence or absence of GM-CSF. During the initial 72 h period, GM-CSF did not influence the percentage of embryos reaching the expanded blastocyst stage, but there was a significant increase (P < 0.05) in the number of cells in the embryos grown with GM-CSF. Survival after thawing was not affected by previous exposure to GM-CSF, but re-expansion of the blastocoele was diminished in that group. Exposure to GM-CSF during the post-thaw period greatly enhanced re-expansion of the blastocoele. The presence of human serum albumin in the culture media is thought to have masked the beneficial effect of GM-CSF upon embryos.

The alpha subunit of the granulocyte-macrophage colony-stimulating factor receptor interacts with c-Kit and inhibits c-Kit signaling

The cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates hematopoiesis and the function of mature host defense cells through the GM-CSF receptor (GMR), which is composed of alpha (alphaGMR) and beta (betaGMR) subunits. Stem cell factor is another important hematopoietic cytokine that signals through c-Kit, a receptor tyrosine kinase, and regulates hematopoietic stem cell maintenance and erythroid development. Like other cytokine receptors, GMR and c-Kit are generally deemed as independent adaptor molecules capable of transducing cytokine-specific signals. We found that the alphaGMR directly interacts with c-Kit and that the interaction is mediated by the cytoplasmic domains. Furthermore, alphaGMR inhibited c-Kit auto-phosphorylation induced by the ligand stem cell factor. Consistent with the inhibitory effect, the expression of alphaGMR was suppressed in cells whose viability was dependent on c-Kit signaling. In contrast, the alternatively spliced alpha2 isoform of the alphaGMR could not inhibit c-Kit signaling, providing a rationale for the existence of the alpha2 isoform. Our results suggest that in addition to having the commonly appreciated roles in cytokine signal transduction, the receptors alphaGMR and c-Kit could interact to coordinate their signal initiation.

Mediterranean diet, olive oil and cancer

Olive oil is an integral ingredient of the "Mediterranean diet" and accumulating evidence suggests that it may have a potential role in lowering the risk of several types of cancers. The mechanisms by which the cancer-preventing effects of olive oil can be performed, however, are not known. We recently hypothesized that a novel molecular explanation concerning the anti-cancer actions of olive oil may relate to the ability of its monounsaturated fatty acid (MUFA) oleic acid (OA; 18:1n-9) to specifically regulate cancer-related oncogenes. Supporting our hypothesis, exogenous supplementation of cultured breast cancer cells with physiological concentrations of OA was found to suppress the overexpression of HER2 (Her-2/neu, erbB-2), a well-characterized oncogene playing a key role in the etiology, progression and response to chemotherapy and endocrine therapy in approximately 20% of breast carcinomas. OA treatment was also found to synergistically enhance the efficacy of trastuzumab, a humanized monoclonal antibody binding with high affinity to the ectodomain (ECD) of the Her2-coded p185(HER2) oncoprotein. Moreover, OA exposure significantly diminished the proteolytic cleavage of the ECD of HER2 and, consequently, its activation status, a crucial molecular event that determines both the aggressive behavior and the response to trastuzumab of Her2-overexpressing breast carcinomas. Our most recent findings further reveal that OA exposure may suppresses HER2 at the transcriptional level by up-regulating the expression of the Ets protein PEA3 -a DNA-binding protein that specifically blocks HER2 promoter activity- in breast, ovarian and stomach cancer cell lines. This anti-HER2 property of OA offers a previously unrecognized molecular mechanism by which olive oil may regulate the malignant behavior of cancer cells. From a clinical perspective, it could provide an effective means of influencing the outcome of Her-2/neu-overexpressing human carcinomas with poor prognosis. Indeed, OA-induced transcriptional repression of HER2 oncogene may represent a novel genomic explanation linking "Mediterranean diet", olive oil and cancer as it seems to equally operate in various types of Her-2/neu-related carcinomas.

Granulocyte-macrophage colony-stimulating factor stimulates mouse blastocyst inner cell mass development only when media lack human serum albumin

The aim of the current study was to examine the effects of granulocyte-macrophage colony-stimulating factor (GM-CSF) on the development and differentiation of preimplantation mouse embryos from different strains and under different culture conditions. Embryos from F1 hybrid mice were cultured in a modified G1 medium lacking amino acids and EDTA (simple G1), human tubal fluid medium (HTF) or in G1/G2 sequential media, supplemented with GM-CSF (0, 2, 4, 8, and 16 ng/ml). Embryos from CF1 mice were subsequently cultured in G1/G2 with (5 mg/ml) or without HSA, in the absence or presence of GM-CSF (2 ng/ml). GM-CSF had no effect at any concentration on F1 embryo development and blastocyst cell numbers, irrespective of the culture media used. Similarly, GM-CSF had no effect on CF1 blastocyst development. However, a stimulatory effect of GM-CSF was evident on total blastocyst cell number and ICM development when CF1 embryos were cultured in the absence of HSA. When HSA was present in the media the beneficial effect of GM-CSF was negated. There was no difference in the number of apoptotic cells in CF1 blastocysts when G1/G2 were supplemented with GM-CSF with or without HSA. These data indicate that there is no beneficial effect of supplementing either simple (simple G1 or HTF) or more complete (G1/G2) media with GM-CSF when protein is present in the medium. However, when culture conditions are suboptimal and non-physiological, i.e. the absence of protein, GM-CSF stimulates development of both total cell numbers and ICM development of CF1 blastocysts.

Dysregulation of granulocyte, erythrocyte, and NK cell lineages in Fli-1 gene–targeted mice

Targeted disruption of the Friend leukemia integration 1 (Fli-1) proto-oncogene results in severe dysmegakaryopoiesis and embryonic lethality. We used morula-stage aggregation as a strategy to further clarify the hematopoietic defects of the Fli-1 gene-targeted mice. Analyses of lineage expression of Fli-1+/- and Fli-1-/- cells in the peripheral blood and bone marrow of chimeric mice consistently demonstrated reduced numbers of neutrophilic granulocytes and monocytes and increased numbers of natural killer (NK) cells. Transplantation studies using sorted Fli-1 mutant cells produced similar findings. Clonal culture studies of bone marrow cells revealed increased numbers of granulocytic and early erythroid progenitors in the Fli-1+/- cells. The sorted Fli-1-/- bone marrow cells revealed specific down-regulation of CCAAT/enhancer binding protein-α (C/EBPα) and C/EBPϵ, and the receptors for granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage CSF (GM-CSF), consistent with their critical roles in granulopoiesis. Collectively, these observations suggest previously unknown physiologic roles for Fli-1 in granulocytic, erythroid, and NK cell proliferation and differentiation. Production of chimeras by morula-stage embryo aggregation is an effective way to unravel cell-autonomous hematopoietic defects in gene-targeted mice.

Short tandem repeats are associated with diverse mRNAs encoding membrane-targeted proteins

Within the genomes of multicellular organisms, short tandem repeating sequences (STRs) are ubiquitous, yet usage patterns remain obscure. The repeats (AC)n and (GU)n appear frequently in the untranslated regions (UTRs) of messenger RNAs (mRNAs). To investigate STR usage patterns, we used three approaches: (1) comparisons of individual mRNA database sequences including annotations and linked references, (2) statistical analysis of complete, UTR databases and (3) study of a large gene family, the aquaporins. Among 500 (AC)n- or (GU)n-containing mRNAs, 58 (12%) had known functions. Of these, 50 (86%) encoded proteins whose activities involved membranes or lipids, including integral membrane proteins, peripheral membrane proteins, ion channels, lipid enzymes, receptors and secreted proteins. A control sequence (AU)n also occurred in mRNAs, but only 5% encoded membrane-related functions. Investigation of all reported 3' UTR sequences, demonstrated that the STR (AC)n was 9 times more common in mRNAs encoding membrane functions than in the total UTR database (P < 0.001). Similarly, (GU)n was 8 times more common in membrane-function mRNAs than in the total database (P < 0.001). These observations suggest that (AC)n and (GU)n may be UTR signals for some mRNAs encoding membrane-targeted proteins.

Granulocyte macrophage colony stimulating factor (GM-CSF) activity is regulated by a GM-CSF binding molecule in Wallerian degeneration following injury to peripheral nerve axons

The hematopoietic factor and inflammatory cytokine GM-CSF is involved in PNS and CNS injury and disease, and in macrophage and microglia function regulation. We presently document that injury to PNS axons induces in vivo production of GM-CSF-inhibitor and GM-CSF-augmenter activities. GM-CSF-inhibitor activity was detected in extract and conditioned medium (CM) of injured PNS but not in extract of intact PNS, and was removed from CM by GM-CSF affinity chromatography, suggesting it is carried by a secreted GM-CSF binding molecule. CM further displayed GM-CSF-augmenter activity along with GM-CSF-inhibitor activity but at contrasting concentrations; augmentation at lowest and inhibition at highest. GM-CSF activity is thus regulated during Wallerian degeneration (WD); augmenter activity characterizes the onset and inhibitor activity the later stages of WD.

Disruption of Smad5 gene leads to enhanced proliferation of high-proliferative potential precursors during embryonic hematopoiesis

SMAD proteins are downstream signal transducers of the transforming growth factor beta (TGF-beta) superfamily, which serve as pleiotropic regulators in embryonic and adult hematopoiesis. SMAD5, initially considered to mediate bone morphogenetic proteins (BMPs) signals, can also transduce the inhibitory signal of TGF-beta1 on proliferation of hematopoietic progenitors derived from human bone marrow. To define its specific role in regulation of primitive multipotential progenitors during early embryonic hematopoiesis, we examined Smad5(-/-) yolk sacs at E9.0 to 9.5 and detected an elevated number of high-proliferative potential colony-forming cells (HPP-CFCs) with enhanced replating potential. To exclude the possible influence of microenvironmental deficit on embryonic hematopoiesis in vivo, we performed in vitro embryonic stem (ES) cell differentiation assay and investigated the HPP-CFCs in particular. Smad5(-/-) embryoid bodies (EBs) contained an elevated number of blast colony-forming cells (BL-CFCs), the in vitro equivalent of hemangioblast, in contrast to reduced proliferation of primitive erythroid precursors (Ery/Ps) within the mutant EBs. More importantly, profoundly increased frequency of HPP-CFCs, featured with a gene-dosage effect, was detected within day 6 Smad5(-/-) EBs compared with the wild type. In addition, Smad5(-/-) HPP-CFCs displayed enhanced self-renewal capacity and decreased sensitivity to TGF-beta1 inhibition, suggesting a critical role of Smad5 in TGF-beta1 regulation of embryonic HPP-CFCs. Consistently, reverse transcription-polymerase chain reaction analysis detected alterations of the transcription factors including GATA-2 and AML1 as well as cytokine receptors in Smad5(-/-) HPP-CFC colonies. Together, these data define an important function of SMAD5 in negative regulation of high-proliferative potential precursors during embryonic hematopoiesis.

Selective delivery of augmented IL-2 receptor signals to responding CD8+ T cells increases the size of the acute antiviral response and of the resulting memory T cell pool

CD8(+) T cells respond to IL-2 produced both endogenously and by CD4(+) Th during an antiviral response. However, IL-2R signals can potentially promote CD8(+) T cell death as well as proliferation, making it unclear whether IL-2R signals provide a predominantly positive or negative effect upon CD8(+) T cell responses to viral infection. To more precisely define the direct role of IL-2R signaling on CD8(+) T cells during the response to a virus, we examined the effect of delivering augmented IL-2R signals selectively to CD8(+) T cells responding to lymphocytic choriomeningitis virus infection. Although naive CD8(+) T cells are competent to produce IL-2, CD8(+) T cells lose this capacity upon differentiation into effector CD8(+) T cells. However, effector CD8(+) T cells do retain the capacity to produce GM-CSF upon Ag stimulation. Thus, to deliver enhanced autocrine IL-2R signals to CD8(+) T cells, we established a transgenic mouse strain expressing a chimeric GM-CSF/IL-2R (GMIL2R). As GM-CSF production is Ag dependent, the GMIL2R delivers an augmented IL-2R signal exclusively to CD8(+) T cells responding to Ag. Following lymphocytic choriomeningitis virus infection, GMIL2R transgenic mice exhibited an increase in both the peak CD8(+) T cell response achieved and the size of the resulting memory pool established. Upon secondary viral challenge, the GMIL2R also enhanced the proliferative response of memory CD8(+) T cells. Thus, our findings indicate that IL-2 delivery to responding CD8(+) T cells is a limiting factor in both the acute and memory antiviral responses.

Granulocyte-macrophage colony-stimulating factor induces an expression program in neonatal microglia that primes them for antigen presentation

Neonatal microglial cells respond to GM-CSF and M-CSF by acquiring different morphologies and phenotypes. To investigate the extent and consequences of this process, a global gene expression analysis was performed, with significant changes in transcript levels confirmed by biochemical analyses. Primary murine microglial cells underwent substantial expression reprogramming after treatment with GM-CSF or M-CSF with many differentially expressed transcripts important in innate and adaptive immunity. In particular, many gene products involved in Ag presentation were induced by GM-CSF, but not M-CSF, thus potentially priming relatively quiescent microglia cells for Ag presentation. This function of GM-CSF is distinct from its primary function in cell proliferation and survival.

Vitamin C inhibits granulocyte macrophage-colony-stimulating factor-induced signaling pathways

Vitamin C is present in the cytosol as ascorbic acid, functioning primarily as a cofactor for enzymatic reactions and as an antioxidant to scavenge free radicals. Human granulocyte macrophage-colony-stimulating factor (GM-CSF) induces an increase in reactive oxygen species (ROS) and uses ROS for some signaling functions. We therefore investigated the effect of vitamin C on GM-CSF-mediated responses. Loading U937 cells with vitamin C decreased intracellular levels of ROS and inhibited the production of ROS induced by GM-CSF. Vitamin C suppressed GM-CSF-dependent phosphorylation of the signal transducer and activator of transcription 5 (Stat-5) and mitogen-activated protein (MAP) kinase (Erk1 and Erk2) in a dose-dependent manner as was phosphorylation of MAP kinase induced by both interleukin 3 (IL-3) and GM-CSF in HL-60 cells. In 293T cells transfected with alpha and beta GM-CSF receptor subunits (alphaGMR and betaGMR), GM-CSF-induced phosphorylation of betaGMR and Jak-2 activation was suppressed by vitamin C loading. GM-CSF-mediated transcriptional activation of a luciferase reporter construct containing STAT-binding sites was also inhibited by vitamin C. These results substantiate the importance of ROS in GM-CSF signaling and indicate a role for vitamin C in downmodulating GM-CSF signaling responses. Our findings point to vitamin C as a regulator of cytokine redox-signal transduction in host defense cells and a possible role in controlling inflammatory responses.

Enhanced signaling through the IL-2 receptor in CD8+ T cells regulated by antigen recognition results in preferential proliferation and expansion of responding CD8+ T cells rather than promotion of cell death

Multiple cytokines, including IL-2, can affect T cell proliferation and survival. However, IL-2 can lead to apoptosis as well as proliferation, making unclear whether IL-2 receptor (IL-2R) signals ultimately have a predominantly positive or negative effect. To address this issue, we examined the effect of enhancing IL-2R signals in CD8(+) T cells after antigen stimulation by engineering a transgenic (Tg) mouse strain with CD8(+) T cells capable of augmented, regulated, autocrine IL-2R signaling after target recognition by means of expression of a chimeric granulocyte-macrophage colony-stimulating factor (GM-CSF)/IL-2R. The Tg CD8(+) T cells can bind the granulocyte-macrophage colony-stimulating GM-CSF produced by antigen stimulation, but the GM-CSF binding results in delivery of an IL-2R signal. After antigen stimulation in vivo, the Tg T cells demonstrated marked increases in the initial proliferative response and cell expansion and displayed continued increases in cell expansion after repeated antigen exposure. These data suggest that the predominant role of IL-2R signals delivered to responding CD8(+) T cells is to set the size of the initial response to antigen by promoting T cell proliferation and survival and not cell death.

Differential divergence of three human pseudoautosomal genes and their mouse homologs: implications for sex chromosome evolution

The human pseudoautosomal region 1 (PAR1) is essential for meiotic pairing and recombination, and its deletion causes male sterility. Comparative studies of human and mouse pseudoautosomal genes are valuable in charting the evolution of this interesting region, but have been limited by the paucity of genes conserved between the two species. We have cloned a novel human PAR1 gene, DHRSXY, encoding an oxidoreductase of the short-chain dehydrogenase/reductase family, and isolated a mouse ortholog Dhrsxy. We also searched for mouse homologs of recently reported PGPL and TRAMP genes that flank it within PAR1. We recovered a highly conserved mouse ortholog of PGPL by cross-hybridization, but found no mouse homolog of TRAMP. Like Csf2ra and Il3ra, both mouse homologs are autosomal; Pgpl on chromosome 5, and Dhrsxy subtelomeric on chromosome 4. TRAMP, like the human genes within or near PAR1, is probably very divergent or absent in the mouse genome. We interpret the rapid divergence and loss of pseudoautosomal genes in terms of a model of selection for the concentration of repetitive recombinogenic sequences that predispose to high recombination and translocation.

Resolution of pluripotential intermediates in murine hematopoietic differentiation by global complementary DNA amplification from single cells: confirmation of assignments by expression profiling of cytokine receptor transcripts

Although hematopoiesis is known to proceed from stem cells through a graded series of multipotent, oligopotent, and unipotent precursor cells, it has been difficult to resolve these cells physically one from another. There is, therefore, corresponding uncertainty about the exact distribution and timing of the expression of genes known to be important in hematopoietic differentiation. In earlier work, the generation of a set of amplified complementary DNAs (cDNAs) from single precursor cells was described, whose biologic potential was determined by the outcome of cultured sibling cells. In this study, the new acquisition of cDNA from multipotent myeloid precursor cells is described, as is the mapping of RNA-level expression of 17 distinct cytokine receptors (c-kit, Flk-1, Flk-2/Flt-3, c-fms, gp130, erythropoietin receptor, GM-CSFRalpha, G-CSFR, TNFR1, IL-1RI, IL-1RII, IL-2Rbeta, IL-3-specific beta receptor, IL-4R, IL-6Ralpha, IL-7Ralpha, and IL-11Ralpha) to the enlarged sample set, spanning stages from pentapotent precursors through oligopotent intermediates to committed and maturing cells in the myeloid and lymphoid lineages. Although the enhanced scope and resolving power of the analysis yielded previously unreported observations, there was overall agreement with known biologic responsiveness at individual stages, and major contradictions did not arise. Moreover, each precursor category displayed a unique overall pattern of hybridization to the matrix of 17 receptor probes, supporting the notion that each sample pool indeed reflected a unique precursor stage. Collectively, the results provide supportive evidence for the validity of the cDNA assignments to particular stages, the depth of the information captured, and the unique capacity of the sample matrix to resolve individual stages in the hematopoietic hierarchy.

Granulocyte-macrophage colony-stimulating factor promotes glucose transport and blastomere viability in murine preimplantation embryos

Granulocyte-macrophage colony-stimulating factor (GM-CSF) secretion from epithelial cells lining the female reproductive tract is induced during early pregnancy by ovarian steroid hormones and constituents of seminal plasma. In this study we have investigated the influence of GM-CSF on development of preimplantation mouse embryos. Blastocyst-stage embryos were found to specifically bind (125)I-GM-CSF and analysis of GM-CSF mRNA receptor expression by reverse transcriptase-polymerase chain reaction indicated expression of the low-affinity alpha subunit of the GM-CSF receptor, but not the affinity-converting beta subunit (beta(c)), or GM-CSF ligand. GM-CSF receptor mRNA was present in the fertilized oocyte and all subsequent stages of development, and in blastocysts it was expressed in both inner cell mass and trophectoderm cells. In vitro culture of eight-cell embryos in recombinant GM-CSF accelerated development of blastocysts to hatching and implantation stages, with a maximum response at a concentration of 2 ng/ml (77 pM). Blastocysts recovered from GM-CSF-null mutant (GM-/-) mice on Day 4 of natural pregnancy or after superovulation showed retarded development, with the total cell number reduced by 14% and 18%, respectively, compared with GM+/+ embryos. Blastocysts generated in vitro from two-cell GM-/- and GM+/+ embryos were larger when recombinant GM-CSF was added to the culture medium (20% and 24% increases in total cell numbers in GM+/+ and GM-/- blastocysts, respectively). Incubation of blastocysts with recombinant GM-CSF elicited a 50% increase in the uptake of the nonmetabolizable glucose analogue, 3-O-methyl glucose. In conclusion, these data indicate that GM-CSF signaling through the low-affinity GM-CSF receptor in blastocysts is associated with increased glucose uptake and enhanced proliferation and/or viability of blastomeres. Together, the findings implicate a physiological role for maternal tract-derived GM-CSF in targeting the preimplantation embryo, and suggest that defective blastocyst development contributes to compromised pregnancy outcome in GM-CSF-null mutant mice.

Antisense inhibition of membrane-bound human interleukin-5 receptor-alpha chain does not affect soluble receptor expression and induces apoptosis in TF-1 cells

Binding of human interleukin-5 (HuIL-5) to its membrane-anchored receptor (IL-5R) triggers multiple signaling pathways, cellular proliferation, and maturational responses, as well as protection from apoptosis. In contrast, soluble forms of the HuIL-5R have been shown to inhibit IL-5 signaling and, therefore, may represent naturally occurring negative regulators of IL-5 function. Because of the central role of IL-5 in promoting eosinophilia and airway hyperresponsiveness in animal models of asthma, antisense oligonucleotides specific either for the membrane form alone or for sequences shared between both the membrane and soluble forms of the HuIL-5Ralpha ligand binding chain were designed. The activities of these oligonucleotides were characterized in IL-5R-expressing erythroleukemic TF-1 cells. Herein we report that an antisense oligonucleotide targeted to a sequence unique to the alternatively spliced membrane-bound form of the HuIL-5Ralpha chain has been developed that selectively inhibits membrane, but not soluble, mRNA isoform expression. Both this membrane-specific oligonucleotide and an antisense oligonucleotide targeted to sequence common to both membrane and soluble isoforms were found to potently suppress cell surface IL-5Ralpha levels and IL-5-mediated cell survival by inducing apoptosis similar to IL-5 withdrawal. Thus, these oligonucleotides represent unique genetic agents with therapeutic potential for diseases with an eosinophilic component.

Transcriptional activation of the granulocyte-macrophage colony-stimulating factor receptor gene in cell mutants

Retroviral insertional mutagenesis has proven to be a powerful in vivo approach for identifying genetic mutations involved in tumorigenesis or developmental abnormalities. Applying this approach to an in vitro system, where experimental design can be readily manipulated, would greatly increase its efficacy. In this study, we sought to determine whether retroviral insertional mutagenesis could be used to isolate cell mutants, in which the transcriptional activation of a receptor gene has occurred. Cells of the myeloid progenitor cell line FDC-P1(M), which do not express the alpha receptor subunit (GMRalpha) for granulocyte-macrophage colony-stimulating factor (GM-CSF), were infected and selected for growth in GM-CSF. Over 100 mutants were isolated at a frequency up to ninefold higher than that of uninfected controls. Expression of GMRalpha in these mutants was confirmed by blocking proliferation with GM-CSF antibodies, detection of high-affinity receptors, and Northern blot analysis. Significantly, in 7/18 mutants analyzed, gross DNA rearrangements had occurred in the GMRalpha locus. These rearrangements were demonstrated to be due to intergenic rearrangements, juxtaposing an active enhancer/promoter upstream of the GMRalpha gene. In one mutant it could be demonstrated that the wild-type allele was also expressed, providing evidence that secondary mutations had occurred. The implications of these results for retroviral insertional mutagenesis are discussed.

Deletion of individual exons and induction of soluble murine interleukin-5 receptor-alpha chain expression through antisense oligonucleotide-mediated redirection of pre-mRNA splicing

Expression of the interleukin-5 receptor-alpha (IL-5Ralpha) chain is thought to play an important role in the pathogenesis of asthma and other eosinophilic diseases. With antisense oligonucleotides (ASOs) chemically modified to provide increased hybridization affinity for RNA but that do not support RNase H-mediated cleavage (2'-O-methoxyethyl-modified ASOs), we show that constitutive splicing of murine IL-5Ralpha mRNA can be modulated in cells such that individual exons may be selectively deleted from mature transcripts. Specific deletion of individual exons and redirection of alternative splicing of the IL-5Ralpha mRNA have been achieved with this approach, by targeting 3'-splice sites or exon sequences immediately downstream of an alternative splice site. ASO targeting with these strategies resulted in inhibition of mRNA and protein levels of the membrane IL-5Ralpha isoform capable of signaling IL-5-mediated growth and antiapoptotic signals to eosinophils. Membrane isoform IL-5Ralpha inhibition was coupled with an increase in expression of mRNA for the alternatively spliced soluble isoform, which binds IL-5 extracellularly and may block its function. These observations suggest the potential general therapeutic use of an antisense approach to increase expression of variant RNA transcripts and to thereby produce proteins devoid of specific functional domains that may impact disease processes, as well as its specific utility for modulating expression of a key cytokine receptor implicated in allergic inflammation.

High-affinity binding to the GM-CSF receptor requires intact N-glycosylation sites in the extracellular domain of the β subunit

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor consists of 2 glycoprotein subunits, GMR and GMRβ. GMR in isolation binds to GM-CSF with low affinity. GMRβ does not bind GM-CSF by itself, but forms a high-affinity receptor in association with GMR. Previously, it was found that N-glycosylation of GMR is essential for ligand binding. The present study investigated the role of N-glycosylation of the β subunit on GM-CSF receptor function. GMRβ has 3 potential N-glycosylation sites in the extracellular domain at Asn58, Asn191, and Asn346. Single mutants and triple mutants were constructed, converting asparagine in the target sites to aspartic acid or alanine. A single mutation at any of the 3 consensus N-glycosylation sites abolished high-affinity GM-CSF binding in transfected COS cells. Immunofluorescence and subcellular fractionation studies demonstrated that all of the GMRβ mutants were faithfully expressed on the cell surface. Reduction of apparent molecular weight of the triple mutant proteins was consistent with loss of N-glycosylation. Intact N-glycosylation sites of GMRβ in the extracellular domain are not required for cell surface targeting but are essential for high-affinity GM-CSF binding.

High-affinity binding to the GM-CSF receptor requires intact N-glycosylation sites in the extracellular domain of the β subunit

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor consists of 2 glycoprotein subunits, GMR and GMRβ. GMR in isolation binds to GM-CSF with low affinity. GMRβ does not bind GM-CSF by itself, but forms a high-affinity receptor in association with GMR. Previously, it was found that N-glycosylation of GMR is essential for ligand binding. The present study investigated the role of N-glycosylation of the β subunit on GM-CSF receptor function. GMRβ has 3 potential N-glycosylation sites in the extracellular domain at Asn58, Asn191, and Asn346. Single mutants and triple mutants were constructed, converting asparagine in the target sites to aspartic acid or alanine. A single mutation at any of the 3 consensus N-glycosylation sites abolished high-affinity GM-CSF binding in transfected COS cells. Immunofluorescence and subcellular fractionation studies demonstrated that all of the GMRβ mutants were faithfully expressed on the cell surface. Reduction of apparent molecular weight of the triple mutant proteins was consistent with loss of N-glycosylation. Intact N-glycosylation sites of GMRβ in the extracellular domain are not required for cell surface targeting but are essential for high-affinity GM-CSF binding.

Cytokine-mediated cPLA(2) phosphorylation is regulated by multiple MAPK family members

Cytosolic phospholipase A(2) (cPLA(2)) plays a critical role in various neutrophil functions including the generation of leukotrienes and platelet-activating factor release. Enzyme activity is regulated both by translocation to the membrane in a Ca(2+)-dependent manner and serine phosphorylation by members of the mitogen-activated protein kinase (MAPK) family. In this report, we have investigated the role of granulocyte/macrophage colony-stimulating factor (GM-CSF)-mediated signalling pathways in the regulation of cPLA(2). GM-CSF-induced cPLA(2) phosphorylation was not affected by pharmacological inhibition of p38 MAPK, phosphatidylinositol 3-kinase or Src. However, inhibition of extracellular signal-regulated kinase (ERK) MAPK activation resulted in a partial inhibition of cPLA(2) phosphorylation, revealed in a slower onset of phosphorylation. A cell line stably transfected with the GM-CSF receptor was used to further analyze GM-CSF-mediated cPLA(2) phosphorylation. Mutation of tyrosine residues 577 and 612 resulted in a delayed cPLA(2) phosphorylation similar to the pharmacological ERK inhibition. Furthermore, inhibition of p38 MAPK in cells bearing the double mutant betac577/612 completely abrogated GM-CSF-induced cPLA(2) phosphorylation. We conclude that GM-CSF can mediate cPLA(2) phosphorylation through the redundant activation of both p38 and ERK MAP kinases.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) targets myeloid leukocytes in the uterus during the post-mating inflammatory response in mice

Factors in seminal plasma elicit a surge of GM-CSF expression in uterine epithelial cells after mating in mice. This study investigates the nature of the endometrial cell populations targeted by epithelial GM-CSF. In quantitative RT-PCR studies, expression of the alpha-subunit of the GM-CSF receptor (GM-CSF-R) parallelled GM-CSF expression, being maximal during the 48 h period after mating and declining thereafter. Expression of mRNA encoding beta-common chain (AIC2B) also increased after mating and remained high until the time of embryo implantation on day 4 of pregnancy. Cells expressing GM-CSF receptors were identified in sections of uterus on the day after mating using 125I-GM-CSF, and were located predominantly in the endometrial stroma subjacent to the luminal epithelium, co-localising with abundant populations of myeloid leukocytes. Cells expressing GM-CSF receptor were identified as macrophages, granulocytes and putative dendritic cells by flow cytometric analysis using lineage and receptor subunit specific antibodies. Recombinant GM-CSF injected into the uterine lumen of ovariectomised mice was found to elicit a dose-dependant accumulation of macrophages and granulocytes in the endometrium, in a pattern of distribution comparable to that seen in uteri after natural mating. Together, these data indicate a role for epithelial cell-derived GM-CSF in mediating the recruitment and potentially in modifying the behaviour of uterine leukocytes during the post-mating inflammatory response in mice.

Orf virus encodes a novel secreted protein inhibitor of granulocyte-macrophage colony-stimulating factor and interleukin-2

The parapoxvirus orf virus encodes a novel soluble protein inhibitor of ovine granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2). The GM-CSF- and IL-2-inhibitory factor (GIF) gene was expressed as an intermediate-late viral gene in orf virus-infected cells. GIF formed homodimers and tetramers in solution, and it bound ovine GM-CSF with a K(d) of 369 pM and ovine IL-2 with a K(d) of 1.04 nM. GIF did not bind human GM-CSF or IL-2 in spite of the fact that orf virus is a human pathogen. GIF was detected in afferent lymph plasma draining the skin site of orf virus reinfection and was associated with reduced levels of lymph GM-CSF. GIF expression by orf virus indicates that GM-CSF and IL-2 are important in host antiviral immunity.

CYRL, a novel cytokine receptor-like protein expressed in testis, lung, and spleen

The interleukin-3 receptor is composed of a ligand-specific alpha subunit (IL-3Ralpha) and a beta subunit (beta(c) or beta(IL3)). Here we report the cloning of a rat brain cDNA transcript with significant homology to IL-3Ralpha, which we have termed CYRL, for CYtokine Receptor-Like protein. A number of conserved motifs identify CYRL as a member of the alpha family of cytokine receptor subunits, but the extracellular domain was too divergent from the mouse IL-3Ralpha sequence to suggest that CYRL is the rat ortholog of IL-3Ralpha. CYRL mRNA expression by Northern blotting was highest in the testis, intermediate in the lung, and modest in spleen, brain, and heart. Antibodies generated against the extracellular domain of CYRL specifically labeled a broad immunoreactive band of M(r) approximately 50,000 in membrane fractions of testis, lung, and spleen. CYRL appears to be a novel cytokine receptor alpha-subunit of unknown function and with no defined ligands.

Kinetic Resolution of Two Mechanisms for High-Affinity Granulocyte-Macrophage Colony-Stimulating Factor Binding to Its Receptor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an important hematopoietic cytokine that exerts its effects by interaction with the GM-CSF receptor (GMR) on the surface of responsive cells. The GM-CSF receptor consists of two subunits: GMR, which binds GM-CSF with low affinity, and GMRβ, which lacks intrinsic ligand-binding capability but complexes with GMR to form a high-affinity receptor (GMR/β). We conducted dynamic kinetic analyses of GM-CSF receptors to define the role of GMRβ in the interaction of ligand and receptor. Our data show that GMR/β exhibits a higher kon than GMR, indicating that GMRβ facilitates ligand acquisition to the binding pocket. Heterogeneity with regard to GM-CSF dissociation from GMR/β points to the presence of loose and tight ligand-receptor complexes in high-affinity binding. Although the loose complex has a koff similar to GMR, the lower koffindicates that GMRβ inhibits GM-CSF release from the tight receptor complex. The two rates of ligand dissociation may provide for discrete mechanisms of interaction between GM-CSF and its high-affinity receptor. These results show that the β subunit functions to stabilize ligand binding as well as to facilitate ligand acquisition.

Kinetic Resolution of Two Mechanisms for High-Affinity Granulocyte-Macrophage Colony-Stimulating Factor Binding to Its Receptor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an important hematopoietic cytokine that exerts its effects by interaction with the GM-CSF receptor (GMR) on the surface of responsive cells. The GM-CSF receptor consists of two subunits: GMR, which binds GM-CSF with low affinity, and GMRβ, which lacks intrinsic ligand-binding capability but complexes with GMR to form a high-affinity receptor (GMR/β). We conducted dynamic kinetic analyses of GM-CSF receptors to define the role of GMRβ in the interaction of ligand and receptor. Our data show that GMR/β exhibits a higher kon than GMR, indicating that GMRβ facilitates ligand acquisition to the binding pocket. Heterogeneity with regard to GM-CSF dissociation from GMR/β points to the presence of loose and tight ligand-receptor complexes in high-affinity binding. Although the loose complex has a koff similar to GMR, the lower koffindicates that GMRβ inhibits GM-CSF release from the tight receptor complex. The two rates of ligand dissociation may provide for discrete mechanisms of interaction between GM-CSF and its high-affinity receptor. These results show that the β subunit functions to stabilize ligand binding as well as to facilitate ligand acquisition.

Molecular determinants of the granulocyte-macrophage colony-stimulating factor receptor complex assembly

The granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GMR) is composed of two chains that belong to the superfamily of cytokine receptors typified by the growth hormone receptor. A common structural element found in cytokine receptors is a module of two fibronectin-like domains, each characterized by seven beta-strands denoted A-G and A'-G', respectively. The alpha-chain (GMRalpha) confers low affinity GM-CSF binding (K(d) = 1-5 nM), whereas the beta-chain (beta(c)) does not bind GM-CSF by itself but confers high affinity binding when associated with alpha (K(d) = 40-100 pM). In the present study, we define the molecular determinants required for ligand recognition and for stabilization of the complex through a convergence of several approaches, including the construction of chimeric receptors, the molecular dynamics of our three-dimensional model of the GM.GMR complex, and site-directed mutagenesis. The functional importance of individual residues was then investigated through ligand binding studies at equilibrium and through determination of the kinetic constants of the GM.GMR complex. Critical to this tripartite complex is the establishment of four noncovalent bonds, three that determine the nature of the ligand recognition process involving residues Arg(280) and Tyr(226) of the alpha-chain and residue Tyr(365) of the beta-chain, since mutations of either one of these residues resulted in a significant decrease in the association rate. Finally, residue Tyr(365) of beta(c) serves a dual function in that it cooperates with another residue of beta(c), Tyr(421) to stabilize the complex since mutation of Tyr(365) and Tyr(421) result in a drastic increase in the dissociation rate (Koff). Interestingly, these four residues are located at the B'-C' and F'-G' loops of GMRalpha and of beta(c), thus establishing a functional symmetry within an apparently asymmetrical heterodimeric structure.

A cell type-specific constitutive point mutant of the common beta-subunit of the human granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5 receptors requires the GM-CSF receptor alpha-subunit for activation

The high affinity receptor for human granulocyte-macrophage colony-stimulating factor (GM-CSF) consists of a cytokine-specific alpha-subunit (hGMRalpha) and a common signal-transducing beta-subunit (hbetac) that is shared with the interleukin-3 and -5 receptors. We have previously identified a constitutively active extracellular point mutant of hbetac, I374N, that can confer factor independence on murine FDC-P1 cells but not BAF-B03 or CTLL-2 cells (Jenkins, B. J., D'Andrea, R. J., and Gonda, T. J. (1995) EMBO J. 14, 4276-4287). This restricted activity suggested the involvement of cell type-specific signaling molecules in the activation of this mutant. We report here that one such molecule is the mouse GMRalpha (mGMRalpha) subunit, since introduction of mGMRalpha, but not hGMRalpha, into BAF-B03 or CTLL-2 cells expressing the I374N mutant conferred factor independence. Experiments utilizing mouse/human chimeric GMRalpha subunits indicated that the species specificity lies in the extracellular domain of GMRalpha. Importantly, the requirement for mGMRalpha correlated with the ability of I374N (but not wild-type hbetac) to constitutively associate with mGMRalpha. Expression of I374N in human factor-dependent UT7 cells also led to factor-independent proliferation, with concomitant up-regulation of hGMRalpha surface expression. Taken together, these findings suggest a critical role for association with GMRalpha in the constitutive activity of I374N.

Specificity of signaling by hematopoietic cytokine receptors: instructive versus permissive effects

The helical cytokines constitute a family of proteins with a common three-dimensional structure. They exert a wide variety of biological effects with a preference for the hematopoietic system. The effects of helical cytokines are mediated by cell surface receptors, which belong to the cytokine receptor superfamily and signal by activating cytoplasmic tyrosine kinases of the Janus kinase (Jak) family and other downstream signaling pathways. The relevance of each of these pathways for eliciting a specific cellular response remains to be determined. This review will focus on cytokine receptors which play a role in the regulation of hematopoiesis and summarize data the address the question how specificity of signaling is achieved.

The soluble granulocyte-macrophage colony-stimulating factor receptor's carboxyl-terminal domain mediates retention of the soluble receptor on the cell surface through interaction with the granulocyte-macrophage colony-stimulating factor receptor beta-subunit

The hematopoietic cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) mediates its activity through binding to cell-surface receptors. The high-affinity GM-CSF receptor (GMR) consists of two transmembrane-anchored subunits: a ligand-specific, low-affinity subunit (GMRalpha); and a signal-transducing beta-subunit (GMRbeta). The human GMRalpha subunit also exists in a soluble isoform (SOLalpha) which antagonizes GM-CSF activity in vitro. Previous studies by us have shown that coexpression of SOLalpha and a mutated GMRbeta in BHK cells results in retention of SOLalpha on the cell surface and the formation of an intermediate affinity binding complex (Kd approximately 300 pM). This paper investigates the mechanism of the retention of SOLalpha on the cell surface. The data demonstrate that SOLalpha is anchored by a direct, ligand-independent interaction with GMRbeta which also occurs when SOLalpha is coexpressed with wild-type GMRbeta. However, SOLalpha and wild-type GMRbeta form a complex which binds GM-CSF with high affinity (Kd = 39 pM), indistinguishable from the binding characteristics of the TMalpha/GMRbeta complex. The experiments further reveal that the interaction between SOLalpha and GMRbeta is abrogated by removal of the unique 16 amino acid carboxyl-terminal domain of SOLalpha. Specific mutation of cysteine 323 in this carboxyl-domain to alanine also eliminates the cell-surface retention of SOLalpha identifying this residue as being necessary for the formation of the SOLalpha/GMRbeta complex.

Altered cytokine (receptor) mRNA expression as a tool in immunotoxicology

Molecular immunotoxicology is aimed at analysing exposure effects on the temporal expression of important immunoregulatory genes. Cytokines play key roles in the immune system and thus molecular immunotoxicology has focused on the analysis of cytokine (expression) levels. These targets offer important new avenues to explore both in terms of mechanistic understanding of immunotoxicity and in terms of developing new assays and tests for predicting the immunotoxic potential of novel compounds. Effects on cytokine levels can be analysed on two different levels, these being mRNA and protein. The choice essentially depends on the aim of the study. Proteins comprise the biological activity so they are a more direct measure than mRNA. mRNA on the other hand, measures at a specific point in time within a tissue or organ, whereas protein is measured in a body fluid, possibly as a spill-over from tissue, or in a supernatant as a summation over a culture period. mRNA levels are assayed using Northern or dot blotting that both comprise hybridisation and using reverse transcription-polymerase chain reaction (RT-PCR). Although the latter technique has both enormous sensitivity and relative ease of operation as important advantages, it requires much more effort in terms of quantitation. References to the nucleic acid sequences of human, murine, and rat cytokines and their receptors are presented (with accession numbers). Examples in which molecular techniques were successfully employed to assess immunotoxicity and (in some cases) understand mechanisms of action are also presented.

Macrophage colony-stimulating factor augments beta-amyloid-induced interleukin-1, interleukin-6, and nitric oxide production by microglial cells

In Alzheimer's disease (AD), a chronic cerebral inflammatory state is thought to lead to neuronal injury. Microglia, intrinsic cerebral immune effector cells, are likely to be key in the pathophysiology of this inflammatory state. We showed that macrophage colony-stimulating factor, a microglial activator found at increased levels in the central nervous system in AD, dramatically augments beta-amyloid peptide (betaAP)-induced microglial production of interleukin-1, interleukin-6, and nitric oxide. In contrast, granulocyte macrophage colony-stimulating factor, another hematopoietic cytokine found in the AD brain, did not augment betaAP-induced microglial secretory activity. These results indicate that increased macrophage colony-stimulating factor levels in AD could magnify betaAP-induced microglial inflammatory cytokine and nitric oxide production, which in turn could intensify the cerebral inflammatory state by activating astrocytes and additional microglia, as well as directly injuring neurons.

PU.1 regulates both cytokine-dependent proliferation and differentiation of granulocyte/macrophage progenitors

PU.1 is a unique regulatory protein required for the generation of both the innate and the adaptive immune system. It functions exclusively in a cell-intrinsic manner to control the development of granulocytes, macrophages, and B and T lymphocytes. We demonstrate that mutation of the PU.1 gene causes a severe reduction in myeloid (granulocyte/macrophage) progenitors. PU.1 -/- myeloid progenitors can proliferate in vitro in response to the multilineage cytokines interleukin-3 (IL-3), IL-6 and stem cell factor but are unresponsive to the myeloid-specific cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF and M-CSF. The failure of PU.1 -/- progenitors to respond to G-CSF is bypassed by transient signaling with IL-3. In the presence of IL-3 and G-CSF, PU.1 -/- progenitors can differentiate into granulocytic precursors containing myeloperoxidase-positive granules. Thus PU.1 is not essential for specification of granulocytic precursors, but is required for their further differentiation. The failure of PU.1 -/- progenitors to respond to M-CSF is due to lack of c-fms gene transcription. Transduction of c-fms into PU.1 -/- myeloid progenitors bypasses the block to M-CSF-dependent proliferation but does not induce detectable macrophage differentiation. Therefore, PU. 1 appears to be essential for specification of monocytic precursors. Importantly, retroviral transduction of PU.1 into mutant progenitors restores responsiveness to myeloid-specific cytokines and development of mature granulocytes and macrophages. Thus PU.1 controls myelopoiesis by regulating both proliferation and differentiation pathways.

The macrosialin promoter directs high levels of transcriptional activity in macrophages dependent on combinatorial interactions between PU.1 and c-Jun

Macrosialin is a transmembrane glycoprotein that is highly expressed in macrophages. In the present studies, macrosialin mRNA levels are shown to be markedly up-regulated during macrophage differentiation of bone marrow progenitor cells in response to macrophage colony-stimulating factor and granulocyte-macrophage colony-stimulating factor. To investigate the mechanisms responsible for regulation of macrosialin expression, we have isolated the macrosialin gene and performed an initial analysis of its transcriptional regulatory elements. The macrosialin promoter and 7.0 kilobase pairs of 5'-flanking information direct high levels of reporter gene activity in monocyte/macrophage-like cells, but little or no expression in nonmyeloid cells. This pattern of expression is dependent on regulatory elements located between -7.0 and -2.5 kilobase pairs from the transcriptional start site that exhibit strong enhancer activity in macrophages and repressor activity in nonmyeloid cells. Analysis of the proximal macrosialin promoter indicates that combinatorial interactions between at least four classes of transcriptional activators, including PU.1/Spi-1 and members of the AP-1 family are required for basal promoter function. PU.1/Spi-1 and c-Jun act synergistically to activate the macrosialin promoter in a nonmyeloid cell line, suggesting that combinatorial interactions between these proteins are involved in regulating macrosialin expression during macrophage differentiation.

Growth factor receptor expression during in vitro differentiation of partially purified populations containing murine stem cells

We have investigated, by semiquantitative RT-PCR, the kinetics of activation of hematopoietic receptors and differentiation markers in partially purified murine hematopoietic stem cells (HSC) induced to differentiate in serum-free culture with combinations of growth factor (GF). The combinations of GF used sustained either multilineage [stem cell factor (SCF) + interleukin 3 (IL-3), or erythroid [SCF + IL-3 + erythropoietin (Epo)] or myeloid [SCF + IL-3 + granulocyte colony-stimulating factor (G-CSF)] differentiation. The GF receptor genes investigated were the alpha and beta subunits of the IL-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor, the erythropoietin receptor, the G-CSF receptor, and c-Fms, the receptor for macrophage colony-stimulating factor (M-CSF). The expression of Gata1 and alpha- and beta-globin was investigated at the same time as a marker of erythroid differentiation. HSC were purified according to standard protocols, which include partitioning of lineage-negative bone marrow cells with the mitochondrial dye Rhodamine 123 (Rho) into Rho-dull (> or = 17% of which reconstitute long-term hematopoiesis in recipient mice) and into Rho-bright (which are as capable as Rho-dull of multilineage differentiation but do not permanently reconstitute the host). The following pattern of expression was observed: the alpha subunit of the IL-3 receptor clearly was expressed in both Rho-bright and Rho-dull cells at the outset, and its expression did not change over time in culture. The beta subunits of the IL-3 and GM-CSF receptor, the alpha subunit of the GM-CSF receptor, the Epo and G-CSF receptors and Fms barely were expressed in purified Rho-bright and Rho-dull cells, but their expression increased in cells cultured both in erythroid and in myeloid GF combinations. Gata1 was expressed maximally in Rho-bright cells but was below the level of detection in Rho-dull cells. Rho-dull cells expressed Gata1 when cultured both in erythroid and in myeloid GF combinations. In contrast, alpha- and beta-globin, which also were not expressed in the purified cells, were induced only in cells stimulated with Epo. These results indicate that the genes for all the GF receptors investigated (with the exception of the alpha subunit of the IL-3 receptor) are expressed at low levels, if any, in purified Rho-bright or Rho-dull cells, but are expressed in their progeny cultured either in erythroid or myeloid GF combinations. The expression of the Epo receptor, in particular, is activated both in erythroid (alpha- and beta-globin positive and in myeloid (alpha- and beta-globin negative) cells. Therefore, activation of the expression of the Epo receptor gene and activation of the erythroid differentiation program are two independent events in normal hematopoiesis.

Receptors That Induce Erythroid Differentiation of Ba/F3 Cells: Structural Requirements and Effect on STAT5 Binding

Ectopic expression of the erythropoietin receptor (EpoR) in the interleukin-3 (IL-3)–dependent cell line Ba/F3 results in growth and partial erythroid differentiation in Epo. In contrast, introduction and activation of the interleukin-5 receptor (IL-5R) or of the granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) results in proliferation only. As this effect is specific to the EpoR, the role of its extracellular or cytoplasmic domain in differentiation was tested after construction of two chimeric receptors. One receptor contained the extracellular domain of EpoR fused to the endodomain of IL-3R β-chain (E/β), while the other contained the EpoR cytoplasmic region fused to the extracellular domain of GM-CSFR α-chain (GMER). Surprisingly, both receptors induced differentiation ruling out a strict specificity of the extracellular or cytoplasmic region of EpoR in this process. Instead the ability to signal differentiation correlated with structural features shared by the EpoR, GMER, and E/β receptors. Dimerization of all three receptors results in the pairing of two signal transducing chains in the cytoplasm, in contrast to the mitogenic receptors IL-3R, IL-5R, GM-CSFR, which assemble as αβ heterodimers. Two new chimeric receptors that fulfilled the structural requirement exemplified by EpoR, but lacked any part of EpoR, were designed to consolidate this model. They consisted of the ectodomains of the GMR-α and IL-5Rα, respectively, fused to the endodomain of IL-3R β-chain. Both receptors were as effective as EpoR in signaling differentiation in response to their cognate ligand. Another property of receptors fulfilling these structural requirements is that they cause a marked delay in signal transducers and activators of transcription 5 (STAT5) activation on ligand stimulation. Taken together our studies show that structural assembly of receptors dictates their potential to signal erythroid differentiation in Ba/F3 cells, that differentiation can take place in the absence of Epo and that a delay in STAT5 activation is highly predictive of this process.

Cytoplasmic domains of the common beta-chain of the GM-CSF/IL-3/IL-5 receptors that are required for inducing differentiation or clonal suppression in myeloid leukaemic cell lines

Granulocyte-macrophage colony stimulating factor (GM-CSF) is a cytokine that controls the production and function of myeloid cells by interaction with a cell surface receptor composed of a specific ligand-binding alpha-chain (hGMRalpha) and a shared signal-transducing beta-chain (beta c). Co-expression of human GMR alpha-chain and wild-type human beta c in two murine leukaemic cell lines (M1 and WEHI-3B D+) conferred the ability to terminally differentiate into macrophages when stimulated with human GM-CSF. Analysis of cytoplasmic truncation mutants of beta c showed that residues to amino acid 783 (numbering from the first amino acid of the leader sequence) were sufficient for the GM-CSF-dependent induction of all aspects of differentiation in both cell types. However, shorter truncations selectively lost, in a cell-specific manner, first the capacity to induce macrophage migration in agar and then cell surface differentiation antigens and clonal suppression of proliferative potential. The data suggest that different aspects of the differentiated phenotype can be dissociated with the required signalling pathways originating from distinct regions of the receptor cytoplasmic domain and cooperating to produce a fully differentiated macrophage. The cooperativity of these pathways and limiting cell signalling intermediate pool sizes could explain the observed cell line differences and may have implications for normal haemopoiesis.

Cytokine Receptor Expression on Hematopoietic Stem and Progenitor Cells

Hematopoietic stem and progenitor cell populations were obtained by fluorescence activated cell sorting of murine bone marrow (BM) cells into Rhodamine-123lo lineage−Ly6A/E+ c-kit+ (primitive stem cells highly enriched for long-term BM repopulating activity), Rhodamine-123med/hi lineage− Ly6A/E+ c-kit+ (mature stem cells highly enriched for short-term BM repopulating activity and day 13 spleen colony-forming activity) and lineage− Ly6A/E− c-kit+ (enriched for in vitro colony forming cells) populations. Neither stem cell population responds to single cytokines in vitro and each requires the synergistic action of two or more cytokines for proliferation, whereas the progenitor cell population proliferates in response to single cytokines. Since each of these cell populations was sorted as c-kit+, they express receptors for stem cell factor. Cell populations were also analyzed by autoradiography for their ability to specifically bind iodinated cytokines and this revealed that both stem cell populations expressed receptors for interleukin-1α (IL-1α), IL-3, IL-6, and granulocyte colony-stimulating factor (G-CSF ), but lacked receptors for macrophage colony-stimulating factor (M-CSF ), granulocyte-macrophage colony stimulating factor (GM-CSF ), and leukemia inhibitory factor (LIF ). Cells within the progenitor cell population specifically bound IL-3, GM-CSF, G-CSF, IL-6, and IL-1α, whereas no receptors were detected for M-CSF and LIF. Within each cell population examined, heterogeneity was observed in the percentage of cells labeled and the number of receptors per cell. These results suggest that stem cell populations can be further subdivided according to their cytokine receptor profile and it will be of interest to determine if such subpopulations have distinctive functional properties.

Cytokine Receptor Expression on Hematopoietic Stem and Progenitor Cells

Hematopoietic stem and progenitor cell populations were obtained by fluorescence activated cell sorting of murine bone marrow (BM) cells into Rhodamine-123lo lineage−Ly6A/E+ c-kit+ (primitive stem cells highly enriched for long-term BM repopulating activity), Rhodamine-123med/hi lineage− Ly6A/E+ c-kit+ (mature stem cells highly enriched for short-term BM repopulating activity and day 13 spleen colony-forming activity) and lineage− Ly6A/E− c-kit+ (enriched for in vitro colony forming cells) populations. Neither stem cell population responds to single cytokines in vitro and each requires the synergistic action of two or more cytokines for proliferation, whereas the progenitor cell population proliferates in response to single cytokines. Since each of these cell populations was sorted as c-kit+, they express receptors for stem cell factor. Cell populations were also analyzed by autoradiography for their ability to specifically bind iodinated cytokines and this revealed that both stem cell populations expressed receptors for interleukin-1α (IL-1α), IL-3, IL-6, and granulocyte colony-stimulating factor (G-CSF ), but lacked receptors for macrophage colony-stimulating factor (M-CSF ), granulocyte-macrophage colony stimulating factor (GM-CSF ), and leukemia inhibitory factor (LIF ). Cells within the progenitor cell population specifically bound IL-3, GM-CSF, G-CSF, IL-6, and IL-1α, whereas no receptors were detected for M-CSF and LIF. Within each cell population examined, heterogeneity was observed in the percentage of cells labeled and the number of receptors per cell. These results suggest that stem cell populations can be further subdivided according to their cytokine receptor profile and it will be of interest to determine if such subpopulations have distinctive functional properties.

Functional replacement of cytokine receptor extracellular domains by leucine zippers

Granulocyte-macrophage colony-stimulating factor receptor signals by a complex which includes the ligand and two different receptor subunits: a low affinity alpha receptor binding chain (granulocyte-macrophage colony-stimulating factor receptor alpha subunit (GM-Ralpha)) and a signal-transducing beta chain (GM-Rbeta). To investigate two unresolved issues in the initiation of signaling, the role of receptor extracellular domains and receptor stoichiometry, we replaced the mouse GM-Ralpha and GM-Rbeta extracellular domains with the leucine zipper domain of either the Fos or Jun molecule. We co-transfected combinations of chimeric receptors into Ba/F3 cells and found that both simple heterodimers of the GM-Ralpha and GM-Rbeta intracellular domains and homodimers of the GM-Rbeta intracellular domain signaled for proliferation. Surprisingly, homodimers of the GM-Ralpha intracellular domain also signaled for prevention of apoptosis in transfected cells. We similarly engineered dimers of the intracellular domain of the human interferon gamma receptor beta subunit and found that homodimers of the intracellular domain signaled for proliferation. When Fos peptide was added to Ba/F3 cells expressing the human interferon gamma receptor beta subunit construct, thereby preventing homodimer formation, the cells no longer proliferated in the absence of mouse interleukin 3.