Identification of a soluble GM-CSF binding protein in the supernatant of a human choriocarcinoma cell line

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.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and inflammatory stimuli up-regulate secretion of the soluble GM-CSF receptor in human monocytes: evidence for ectodomain shedding of the cell surface GM-CSF receptor alpha subunit

Soluble GM-CSF receptor alpha subunit (sGMRalpha) is a soluble isoform of the GMRalpha that is believed to arise exclusively through alternative splicing of the GMRalpha gene product. The sGMRalpha mRNA is expressed in a variety of tissues, but it is not clear which cells are capable of secreting the protein. We show here that normal human monocytes, but not lymphocytes, constitutively secrete sGMRalpha. Stimulation of monocytes with GM-CSF, LPS, PMA, or A23187 rapidly up-regulates the secretion of sGMRalpha in a dose-dependent manner, demonstrating that secretion is also regulated. To determine whether sGMRalpha arose exclusively through alternative splicing of the GMRalpha gene product, or whether it could also be generated through ectodomain shedding of GMRalpha, we engineered a murine pro-B cell line (Ba/F3) to express exclusively the cDNA for cell surface GMRalpha (Ba/F3.GMRalpha). The Ba/F3.GMRalpha cell line, but not the parental Ba/F3 cell line, constitutively shed a sGMRalpha-like protein that bound specifically to GM-CSF, was equivalent in size to recombinant alternatively spliced sGMRalpha (60 kDa), and was recognized specifically by a mAb raised against the ectodomain of GMRalpha. Furthermore, a broad-spectrum metalloprotease inhibitor (BB94) reduced constitutive and PMA-, A23187-, and LPS-induced secretion of sGMRalpha by monocytes, suggesting that shedding of GMRalpha by monocytes may be mediated in part through the activity of metalloproteases. Taken together, these observations demonstrate that sGMRalpha is constitutively secreted by monocytes, that GM-CSF and inflammatory mediators up-regulate sGMRalpha secretion, and that sGMRalpha arises not only through alternative splicing but also through ectodomain shedding of cell surface GMRalpha.

Determinants of the functional interaction between the soluble GM-CSF receptor and the GM-CSF receptor beta-subunit

The GM-CSF receptor consists of a GM-CSF specific low affinity alpha-subunit (GMRalpha) and a beta-subunit (betac) that associates with GMRalpha in the presence of GM-CSF to form a high-affinity complex. A splice variant soluble isoform of GMRalpha (solalpha) consists of the extracellular domain of GMRalpha and a unique 16-amino acid C-terminal domain. Exogenously administered solalpha is unable to associate with betac on the cell surface either in the presence or absence of GM-CSF. However, paradoxically, co-expression of solalpha with betac results in the ligand-independent association of solalpha with betac on the cell surface via the C-terminal domain of solalpha. To study the interaction and functional characteristics of the solalpha-betac complex we engineered a soluble betac-subunit (ECDbeta) and expressed it alone and with solalpha. Co-expressed but not independent sources of solalpha and ECDbeta could be co-precipitated in the absence of ligand demonstrating the extracellular domain of betac was sufficient for association with solalpha upon co-expression. However, independent sources of solalpha could associate with ECDbeta in the presence of GM-CSF as could a C-terminal deficient solalpha mutant (ECDalpha) and the addition of ECDbeta to ECDalpha and GM-CSF was associated with a conversion from a low- to high-affinity ligand-receptor complex.

Identification of the soluble granulocyte-macrophage colony stimulating factor receptor protein in vivo

On the basis of the finding of alternatively spliced mRNAs, the -subunit of the receptor for GM-CSF is thought to exist in both a membrane spanning (tmGMR) and a soluble form (solGMR). However, only limited data has been available to support that the solGMR protein product exists in vivo. We hypothesized that hematopoietic cells bearing tmGMR would have the potential to also produce solGMR. To test this hypothesis we examined media conditioned by candidate cells using functional, biochemical, and immunologic means. Three human leukemic cell lines that express tmGMR (HL60, U937, THP1) were shown to secrete GM-CSF binding activity and a solGMR-specific band by Western blot, whereas a tmGMR-negative cell line (K562) did not. By the same analyses, leukapheresis products collected for autologous and allogeneic stem cell transplants and media conditioned by freshly isolated human neutrophils also contained solGMR. The solGMR protein in vivo displayed the same dissociation constant (Kd = 2-5 nmol) as that of recombinant solGMR. A human solGMR ELISA was developed that confirmed the presence of solGMR in supernatant conditioned by the tmGMR-positive leukemic cell lines, hematopoietic progenitor cells, and neutrophils. Furthermore, the ELISA demonstrated a steady state level of solGMR in normal human plasma (36 ± 17 pmol) and provided data suggesting that plasma solGMR levels can be elevated in acute myeloid leukemias.

Production of Soluble Granulocyte Colony-Stimulating Factor Receptors from Myelomonocytic Cells

It has been speculated that a soluble form of G-CSFR might be physiologically present in humans, since G-CSFR mRNA that lacks a transmembrane domain has been identified from a human myelomonocytic cell line. Here, we demonstrate human soluble G-CSFR (sG-CSFR) of two different molecular sizes (80 and 85 kDa) on an immunoblot analysis using Abs generated against the amino-terminal, extracellular domain of the full-length G-CSFR. Both isoforms of sG-CSFR were able to bind recombinant human G-CSF (rhG-CSF). RT-PCR analysis with primers targeted outside of the transmenbrane region revealed that membrane-anchored G-CSFR is expressed at all maturation stages of purified myeloid cells, including CD34+CD13+ cells (blasts), CD11b−CD15+ cells (promyelocytes or myelocytes), CD11b+CD15+ cells (metamyelocytes and mature neutrophils), and CD14+ cells (monocytes). On the other hand, sG-CSFR mRNA was detectable in CD11b−CD15+, CD11b+CD15+, and CD14+ cells, but not in the CD34+CD13+ blast population. The serum concentration of both isoforms of sG-CSFR appeared to be correlated with the numbers of neutrophils/monocytes before and after rhG-CSF treatment in normal individuals. Thus, two isoforms of sG-CSFR are physiologically secreted from relatively mature myeloid cells and might play an important role in myelopoiesis through their binding to serum G-CSF.

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.

Construction and binding kinetics of a soluble granulocyte-macrophage colony-stimulating factor receptor alpha-chain-Fc fusion protein

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activity is mediated by a cellular receptor (GM-CSFR) that is comprised of an alpha-chain (GM-CSFRalpha), which specifically binds GM-CSF, and a beta-chain (betac), shared with the interleukin-3 and interleukin-5 receptors. GM-CSFRalpha exists in both a transmembrane (tmGM-CSFRalpha) and a soluble form (sGM-CSFRalpha). We designed an sGM-CSFRalpha-Fc fusion protein to study GM-CSF interactions with the GM-CSFRalpha. The construct was prepared by fusing the coding region of the sGM-CSFRalpha with the CH2-CH3 regions of murine IgG2a. Purified sGM-CSFRalpha-Fc ran as a monomer of 60 kDa on reducing SDS-polyacrylamide gel electrophoresis but formed a trimer of 160-200 kDa under nonreducing conditions. The sGM-CSFRalpha-Fc bound specifically to GM-CSF as demonstrated by standard and competitive immunoassays, as well as by radioligand assay with 125I-GM-CSF. The sGM-CSFRalpha-Fc also inhibited GM-CSF-dependent cell growth and therein is a functional antagonist. Kinetics of sGM-CSFRalpha-Fc binding to GM-CSF were evaluated using an IAsys biosensor (Affinity Sensors, Paramus, NJ) with two assay systems. In the first, the sGM-CSFRalpha-Fc was bound to immobilized staphylococcal protein A on the biosensor surface, and binding kinetics of GM-CSF in solution were determined. This revealed a rapid koff of 2.43 x 10(-2)/s. A second set of experiments was performed with GM-CSF immobilized to the sensor surface and the sGM-CSFRalpha-Fc in solution. The dissociation rate constant (koff) for the sGM-CSFRalpha-Fc trimer from GM-CSF was 1.57 x 10(-3)/s, attributable to the higher avidity of binding in this assay. These data indicate rapid dissociation of GM-CSF from the sGM-CSFRalpha-Fc and suggest that in vivo, sGM-CSFRalpha may need to be present in the local environment of a responsive cell to exert its antagonist activity.

Membrane-associated and soluble granulocyte/macrophage-colony-stimulating factor receptor alpha subunits are independently regulated in HL-60 cells

The effects of granulocyte/macrophage-colony-stimulating factor (GM-CSF) are mediated by interaction with its composite receptor (GMR), which consists of a unique alpha subunit (GMR alpha) and a beta subunit (GMR beta) that is common to the receptors for GM-CSF, interleukin 3, and interleukin 5. GMR beta is required for high-affinity binding, cell proliferation, and protein phosphorylation but has no intrinsic GM-CSF-binding activity. GMR alpha in isolation binds to GM-CSF with low affinity and can signal for increased glucose uptake. In addition to the membrane-bound receptor (mGMR alpha), there is a naturally occurring soluble isoform (sGMR alpha) that is released free into the pericellular milieu. Analysis of genomic sequences reveals that the soluble GMR alpha isoform comes about by alternative mRNA splicing. To examine GMR alpha expression, we developed a quantitative reverse transcription-polymerase chain reaction assay based on serial dilutions of in vitro transcribed GMR alpha RNA. This assay provides a strict log-log measure of GMR alpha RNA expression, distinguishes transcripts related to the soluble and membrane-associated isoforms, and quantitatively detects 0.1 fg of GMR alpha-related mRNA. There was little or no GMR alpha expression in two human lymphoid cell lines and in the erythroblastic leukemia cell line K562, but all myeloid cell lines tested expressed both the membrane-associated and soluble isoforms of GMR alpha. Baseline level of expression of both isoforms varied > 20-fold among the myeloid cell lines studied. Differentiation of HL-60 cells to neutrophils with dimethyl sulfoxide led to a 2-fold downregulation of sGMR alpha and a 20-fold upregulation of mGMR alpha. These differentiation-induced transcriptional changes were unrelated to changes in mRNA stability. These findings indicate that sGMR alpha is differentially expressed from mGMR alpha in human hematopoietic cells and that programmed downregulation of sGMR alpha may be important in myeloid maturation.

Molecular cloning of a soluble form of the granulocyte-macrophage colony-stimulating factor receptor alpha chain from a myelomonocytic cell line. Expression, biologic activity, and preliminary analysis of transcript distribution

OBJECTIVE

To analyze the molecular and functional characteristics of a soluble form of the granulocyte-macrophage colony-stimulating factor receptor alpha chain (sGM-CSFR alpha), and analyze transcript expression in immune cells and the cellular constituents of rheumatoid arthritis synovial tissue.

METHODS

We amplified, cloned, and expressed the sGM-CSFR alpha and transmembrane form of the receptor (tmGM-CSFR alpha) from complementary DNA derived from a human myelomonocytic cell line. Competitive polymerase chain reaction assays were developed to determine the absolute and relative amounts of tmGM-CSFR alpha versus sGM-CSFR alpha message synthesized by various cell lines and tissues.

RESULTS

sGM-CSFR alpha transcripts were detected in bone marrow, monocyte/macrophages (cultured in GM-CSF), rheumatoid synovial tissue, and rheumatoid synovial tissue T cell lines, and represented the predominant transcript in synovial fibroblasts and osteoarthritis synovial tissue. Levels of expression in monocyte/macrophages and some synovial fibroblast and T cell lines approached those seen in transfected cell lines producing functional sGM-CSFR alpha.

CONCLUSION

sGM-CSFR alpha represents a functional antagonist of GM-CSF activity in vitro. Expression of sGM-CSFR alpha in bone marrow, rheumatoid synovial tissue T cells, and synovial fibroblasts suggests an important role in vivo, both in regulating myelopoiesis and in modulating the immune response.

Down-regulation of cytokine action

The study of cytokines that regulate all areas of cellular communication has expanded over the past few years. The control and modulation of the complex network of cytokine action remains an area of intense interest. Agents that will modulate cytokine signal transduction at the cellular level will assist in the understanding of the molecular basis of cytokine cellular activation and in the design of drugs for the management of clinical disease. Recent work has demonstrated the existence of complex mechanisms of negative regulation of cytokine action. New methods utilizing isolated protein products that participate in immunomodulation may prove useful for clinical regulation of the host response to cytokine up-regulation. Currently, most interest in soluble cytokine receptors, natural cytokine inhibitors, genetically engineered cytokine antagonists and single or combinations of anti-inflammatory cytokines has focused on the possibility that they may become standard pharmacological agents for the treatment of inflammatory complications of clinical disease. Specifically, TNF and IL-1 inhibitors and the cytokines IL-10 and TGF-beta, alone or in combination may be effective for the inhibition of severe clinical inflammation. Soluble receptors for other cytokines such as IL-6 may prove to be carrier proteins that enhance cytokine action and will require cautious investigation. Because most cytokines are pleiomorphic in their activities, down-regulation through the utilization of direct inhibitors or anti-inflammatory cytokines may cause immunosuppression, making the host susceptible to opportunistic infection. Selective and short-term inhibition of inflammatory cytokine action may be necessary to prevent unwanted clinical side-effects.

Differently spliced cDNAs of human leukocyte tyrosine kinase receptor tyrosine kinase predict receptor proteins with and without a tyrosine kinase domain and a soluble receptor protein

Leukocyte tyrosine kinase (LTK) is a tyrosine kinase that has been suggested to be specific for hematopoietic cells and neuronal cells and reported as an unusual membrane protein lacking an extracellular domain. Here we report the cloning of a human LTK cDNA clone containing the complete open reading frame of a putative receptor tyrosine kinase protein. The extracellular domain of the receptor protein is larger than previously predicted. Furthermore, we have cloned a set of cDNAs representing differently spliced human LTK mRNAs. These cDNAs predict a truncated receptor protein lacking the tyrosine kinase domain and a soluble receptor protein that has neither a transmembrane nor a tyrosine kinase domain. Our results suggest that the LTK gene produces not only the putative receptor tyrosine kinase for unknown ligand but also multiple protein products that may have different functions.

Cytokines in rodent reproduction and the cytokine-endocrine interaction

Insights derived from recent studies employing rodent models demonstrate that the synthesis of pluripotent cytokines is an important function of resident cells in the female reproductive tract. Through steroid hormone regulated secretion of these mediators, resident cells appear to coordinate the recruitment and action of leukocytes that are centrally implicated in the dramatic remodelling processes characteristic of reproductive events.