Hemopoietic growth factors and their interactions with specific receptors

Identification and in vitro characterization of novel nanobodies against human granulocyte colony-stimulating factor receptor to provide inhibition of G-CSF function

It has been shown that Granulocyte colony-stimulating factor (G-CSF) has a higher expression in malignant tumors, and anti-G-CSF therapy considerably decreases tumor growth, tumor vascularization and metastasis. Thus, blocking the signaling pathway of G-CSF could be beneficial in cancer therapy. This study is aimed at designing and producing a monoclonal nanobody that could act as an antagonist of G-CSF receptor. Nanobodies are the antigen binding fragments of camelid single-chain antibodies, also known as VHH. These fragments have exceptional properties which makes them ideal for tumor imaging and therapeutic applications. We have used our previously built nanobody phage libraries to isolate specific nanobodies to the G-CSF receptor. After a series of cross-reactivity and affinity experiments, two unique nanobodies were selected for functional analysis. Proliferation assay, real-time PCR and immunofluorescence assays were used to characterize these nanobodies. Finally, VHH26 nanobody that was able to specifically bind G-CSF receptor (G-CSF-R) on the surface of NFS60 cells and efficiently block G-CSF-R downstream signaling pathway in a dose-dependent manner was selected. This nanobody could be further developed into a valuable tool in tumor therapy and it forms a basis for additional studies in preclinical animal models.

NOD/SCID mice engineered to express human IL-3, GM-CSF and Steel factor constitutively mobilize engrafted human progenitors and compromise human stem cell regeneration

Transplantation of immunodeficient mice with human hematopoietic cells has greatly facilitated studies of the earliest stages of human hematopoiesis. These include demonstration of the ability of injected 'human-specific' hematopoietic growth factors to enhance the production of human cells at multiple levels of differentiation. In contrast, the effects of continuous exposure to such molecules have not been well investigated. Here, we show that nonobese diabetic severe combined immunodeficiency mice genetically engineered to produce ng/ml serum levels of human interleukin-3 (IL-3), granulocyte/macrophage-stimulating factor (GM-CSF) and Steel factor (SF) display a complex phenotype when transplanted with primitive human bone marrow (BM) or fetal liver cells. This phenotype is characterized by an enhancement of terminal human myelopoiesis and a matched suppression of terminal human erythropoiesis, with a slight reduction in human B-lymphopoiesis in the BM of the engrafted mice. Human clonogenic progenitors are more prevalent in the blood of the transplanted growth factor-producing mice and this is accompanied by a very marked reduction of more primitive human cells in the BM. Our findings suggest that long-term exposure of primitive human hematopoietic cells to elevated levels of human IL-3, GM-CSF and SF in vivo may deleteriously affect the stem cell compartment, while expanding terminal myelopoiesis.

Identification of ligand-binding site III on the immunoglobulin-like domain of the granulocyte colony-stimulating factor receptor

The granulocyte colony-stimulating factor receptor (G-CSF-R) forms a tetrameric complex with G-CSF containing two ligand and two receptor molecules. The N-terminal Ig-like domain of the G-CSF-R is required for receptor dimerization, but it is not known whether it binds G-CSF or interacts elsewhere in the complex. Alanine scanning mutagenesis was used to show that residues in the Ig-like domain of the G-CSF-R (Phe(75), Gln(87), and Gln(91)) interact with G-CSF. This binding site for G-CSF overlapped with the binding site of a neutralizing anti-G-CSF-R antibody. A model of the Ig-like domain showed that the binding site is very similar to the viral interleukin-6 binding site (site III) on the Ig-like domain of gp130, a related receptor. To further characterize the G-CSF-R complex, exposed and inaccessible regions of monomeric and dimeric ligand-receptor complexes were mapped with monoclonal antibodies. The results showed that the E helix of G-CSF was inaccessible in the dimeric but exposed in the monomeric complex, suggesting that this region binds to the Ig-like domain of the G-CSF-R. In addition, the N terminus of G-CSF was exposed to antibody binding in both complexes. These data establish that the dimerization interface of the complete receptor complex is different from that in the x-ray structure of a partial complex. A model of the tetrameric G-CSF.G-CSF-R complex was prepared, based on the viral interleukin-6.gp130 complex, which explains these and previously published data.

Granulocyte colony-stimulating factor induces Egr-1 up-regulation through interaction of serum response element-binding proteins

Granulocyte colony-stimulating factor (G-CSF) stimulates the proliferation and maturation of myeloid progenitor cells both in vitro and in vivo. We showed that G-CSF rapidly and transiently induces expression of egr-1 in the NFS60 myeloid cell line. Transient transfections of NFS60 cells with recombinant constructs containing various deletions of the human egr-1 promoter identified the serum response element (SRE) between nucleotides (nt) -418 and -391 as a critical G-CSF-responsive sequence. The SRE (SRE-1) contains a CArG box, the binding site for the serum response factor (SRF), which is flanked at either side by an ETS protein binding site. We demonstrated that a single copy of the wild-type SRE-1 in the minimal promoter plasmid, pTE2, is sufficient to induce transcriptional activation in response to G-CSF and that both the ETS protein binding site and the CArG box are required for maximal transcriptional activation of the pTE2-SRE-1 construct. In electromobility shift assays using NFS60 nuclear extracts, we identified SRF and the ETS protein Fli-1 as proteins that bind the SRE-1. We also demonstrated through electrophoretic mobility shift assays, using an SRE-1 probe containing a CArG mutation, that Fli-1 binds the SRE-1 independently of SRF. Our data suggest that SRE-binding proteins potentially play a role in G-CSF-induced egr-1 expression in myeloid cells.

Identification of a ligand-binding site on the granulocyte colony-stimulating factor receptor by molecular modeling and mutagenesis

Granulocyte colony-stimulating factor (G-CSF) initiates its effects on cells of the neutrophil lineage by inducing formation of a homodimeric receptor complex. The structure of the G-CSF receptor has not yet been determined, therefore we used molecular modeling to identify regions of the receptor that were likely to be involved in ligand binding. The G-CSF receptor sequence was aligned with all the available sequences of the gp130 and growth hormone receptor families and a model of the cytokine receptor homologous domain was constructed, based on the growth hormone receptor structure. Alanine substitution mutagenesis was performed on loops and individual residues that were predicted to bind ligand. Mutant receptors were expressed in factor-dependent Ba/F3 cells and assessed for proliferation response and ligand binding. Six residues were identified that significantly reduced receptor function, with Arg288 in the F'-G' loop having the greatest effect. These residues formed a binding face on the receptor model resembling the growth hormone receptor site, which suggests that the model is reasonable. However, electrostatic analysis of the model provided further evidence that the mechanism of receptor dimerization is different from that of the growth hormone receptor.

Neutralising antibodies to the granulocyte colony-stimulating factor receptor recognise both the immunoglobulin-like domain and the cytokine receptor homologous domain

To define regions of the granulocyte colony-stimulating factor (G-CSF) receptor that are important for ligand binding, neutralising monoclonal antibodies to the human receptor have been produced. Eleven antibodies recognised six different receptor epitopes. Antibodies from three of the epitope groups were able to detect the receptor by western blotting but did not inhibit G-CSF binding. The other three antibody groups inhibited G-CSF binding either completely (groups 1 and 2) or partially (group 3). All the antibodies inhibited proliferation of BA/F3 cells expressing the G-CSF receptor to varying extents. By using human-marine chimeric receptors, the binding sites of the antibodies were mapped to the immunoglobulin-like domain (groups 1 and 3), the cytokine receptor homologous domain (group 2) or the fibronectin type III domains (groups 4 to 6). These results show that the immunoglobulin-like and cytokine receptor homologous domains of the receptor are important for ligand binding and subsequent signalling.

Schistosoma mansoni: peritoneal plasmacytogenesis and polypoid transformation of mesenteric milky spots in infected mice

We have studied inflammatory reactions in the mesenteric tissue of mice infected with Schistosoma mansoni. Perivascular tissue contained diffuse infiltrates of macrophages, eosinophilic granulocytes and lymphocytes. Angiogenesis in the perivascular adipose tissue was associated with superficial plasmacytogenic foci. Polypoid structures were occasionally formed adjacent to inflammatory foci in the adipose tissue, organized around loops of capillaries, with terminal formation of a glomerular capillary network embedded in connective tissue, covered by plasmacytes. We conclude that these structures are specialized milky spots dedicated to active plasmacytogenesis and antibody secretion into the peritoneal cavity of schistosome-infected mice.

What's new in osteoclast ontogeny?

The osteoclast (OC) is a multinuclear bone-resorbing cell which shares several characteristics with cells of the mononuclear phagocyte system. Unlike terminally differentiated macrophages, OCs possess specialized characteristics such as tartrate resistant acid phosphatase activity and the presence of calcitonin receptors. It appears that myeloid progenitor cells, probably granulocyte-macrophage colony-forming units, generate OC precursors which then differentiate and fuse into OCs under the regulation of osteotropic hormones, cytokines and other local factors. Parathyroid hormone and 1,25 dihydroxy Vitamin D3 induce both the formation and fusion of OC precursors, while calcitonin inhibits fusion. Osteoblasts also produce factor(s) which regulate OC precursor differentiation and therefore bone resorption; the nature of these factor(s), however, is unknown. In addition, the OC surface interacts specifically with a range of cellular and extracellular matrix-associated ligands which influence OC differentiation. The precise regulation of OC formation, however, is complex and awaits further investigation.

Monoclonal antibodies that recognize human granulocyte-macrophage colony-stimulating factor and neutralize its bioactivity in vitro

We have produced monoclonal antibodies to bacterially synthesized, human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) and have studied in detail the characteristics of three strongly neutralizing antibodies. The antibodies reacted with GM-CSF at high dilution (EC50 = 0.1-1.7 nM) in an indirect ELISA but did not react with murine GM-CSF or other cytokines. They also recognized glycosylated hGM-CSF produced by human lymphocytes. The antibodies were able to immunoprecipitate rhGM-CSF, but only reacted weakly with rhGM-CSF on Western blots, indicating that they recognized a conformation-dependent epitope. Cross-blocking studies showed that the three antibodies recognized overlapping epitopes. The antibodies inhibited binding of 125I-labeled rhGM-CSF to HL-60 cells at nanomolar concentrations and neutralized GM-CSF activity in two different bioassays. These antibodies thus provide a useful tool for analyzing the specificity of bioassays and for further studies of the production and function of GM-CSF in vitro and in vivo.

Conditioned medium from activated rat macrophages and the recombinant factors, IL-1 beta and GM-CSF, enhance the accessory activity of dendritic cells

Low density lymph node cells (LD-LNC; 5% of total unfractionated LNC) contain 95% of the accessory activity required for responses of T lymphocytes to mitogens. Significantly greater responses to mitogens occur when T lymphocytes are added to LD-LNC that have been exposed overnight to silica, in comparison to responses occurring with LD-LNC incubated without silica. Conditioned medium (CM) from silica-treated LD-LNC is itself able to mediate enhanced responses; i.e., when LD-LNC are exposed overnight to CM alone and mitogen-treated T lymphocytes added the next day. The enhancing activity found in CM from LD-LNC exposed to silica is produced by macrophages; however, their low accessory activity is not enhanced by CM. In contrast, dendritic cells isolated from LD-LNC exposed to silica or to CM show significantly increased accessory activity, but dendritic cells do not produce the enhancing activity found in CM. CM lacks IL-2 activity and does not have any effect on the responses of untreated or mitogen-treated T lymphocytes alone. Thus, macrophages produce the enhancing activity and dendritic cells respond to it. Maximum enhancement of dendritic cell accessory activity requires overnight exposure to CM; once induced, accessory activity is not further modulated after continued incubation in the presence or absence of CM. LD-LNC, adherent peritoneal exudate cells, and adherent thioglycollate-induced peritoneal exudate cells produce enhancing activity after exposure to silica, LPS, and silica plus LPS. After gel filtration of a CM produced by silica plus LPS, enhancing activity shows a broad molecular weight distribution between 20 and 55 kD. IL-1 is present in CM and shows a more narrow molecular weight distribution that falls within the lower molecular weight range for enhancing activity. Silica treatment by itself produces CM containing little IL-1, but abundant enhancing activity; gel filtration of this CM shows that the distribution of enhancing activity is confined more narrowly to the higher molecular weight range, suggesting that IL-1 is one of several factors that enhances the accessory activity of dendritic cells. Recombinant human IL-1 beta does have enhancing activity, but of the other recombinant factors tested only mouse GM-CSF also has enhancing activity. Human IL-1 alpha, tumor necrosis factor alpha, IL-4, rat IL-3 and rat IFN-gamma, as well as L cell-conditioned medium containing M-CSF, lack enhancing activity.

The effects of dose and route of administration on the pharmacokinetics of granulocyte-macrophage colony-stimulating factor

The pharmacokinetics of granulocyte-macrophage colony stimulating factor (GM-CSF) (0.3-30 micrograms/kg) were studied after subcutaneous bolus (n = 16) or intravenous bolus (n = 5) injection or 2 h intravenous infusion (n = 12). Each method of administration gave a different GM-CSF concentration-time profile. Highest peak serum concentrations (Cmax) followed the intravenous bolus, and the time GM-CSF persisted at a concentration greater than 1 ng/ml (t greater than 1 ng/ml) was longer after a subcutaneous than after an intravenous injection. Area under the concentration-time curve (AUC), Cmax and t greater than 1 ng/ml all increased with dose for each method of administration. After intravenous administration, there was a two-phase decline in concentration. The half-life (t1/2) of the terminal phase following an intravenous bolus ranged from 0.24 to 1.18 h and, following intravenous infusion, from 0.62 to 9.07 h and appeared to increase with dose. The apparent clearance was greatest following subcutaneous injection at doses below 3 micrograms/kg, suggesting a saturable mechanism or different bioavailability. Only 0.001%-0.2% of the injected dose appeared in the urine as immunoreactive GM-CSF.

The role of growth-factor receptors (excluding IL-2 receptors) in the proliferation and differentiation of normal and leukemic hematopoietic cells

Receptors (R) are considered as allosteric enzymes whose action on metabolic chains is modulated through binding to the ligand. They play an essential role in the transduction of the multiple signals (e.g. interleukins or CSF) which intervene in the regulation of hematopoiesis. Their ordered interactions are necessary to regulate the growth and differentiation of normal hematopoietic precursors. This paper summarizes recent data concerning the structure-action relationship of growth-factor receptors in the signal transduction and alterations of growth-factor receptors which may play an important role in leukemic transformation. Some therapeutic modulations of growth-factors cascades are also discussed.

Effects of interleukin 3 and of granulocyte-macrophage and macrophage colony stimulating factors on osteoclast differentiation from mouse hemopoietic tissue

The effects of granulocyte-macrophage colony stimulating factor (GM-CSF), macrophage colony stimulating factor (M-CSF), and interleukin 3 (IL3) on osteoclast formation were tested by incubation of murine hemopoietic cells on plastic coverslips and bone slices with GM-CSF, M-CSF, or IL3, with or without 1,25(OH)2 vitamin D3 (1,25(OH)2D3). Osteoclastic differentiation was detected after incubation by scanning electron microscopical examination of bone slices for evidence of osteoclastic excavations, and by autoradiographic assessment of cells for 1,25(OH)2D3-calcitonin (CT) binding. The differentiation of CT-receptor-positive cells preceded bone resorption, but the number that developed correlated with the extent of bone resorption (r = 0.88). M-CSF and GM-CSF substantially reduced bone resorption and CT-receptor-positive cell formation. The degree of inhibition of bone resorption could not be attributed to effects on the function of mature cells, since M-CSF inhibits resorption by such cells only by 50%, and GM-CSF has no effect. GM-CSF inhibited the development of mature function (bone resorption) to a greater extent than it inhibited CT-receptor-positive cell formation. Since CT-receptor expression antedated resorptive function, this suggests that GM-CSF resulted in the formation of reduced numbers of relatively immature osteoclasts. This suggests that it may exert a restraining effect on the maturation of cells undergoing osteoclastic differentiation in response to 1,25(OH)2D3. Conversely, IL3, which also has no effect on mature osteoclasts, by itself induced CT-receptor expression but not bone resorption; in combination with 1,25(OH)2D3 it induced a threefold increase in bone resorption and CT-receptor-positive cells compared with cultures incubated with 1,25(OH)2D3 alone. IL3 did not induce CT-receptors in peritoneal macrophages, blood monocytes, or J 774 cells. The results suggest that IL3 induces only partial maturation of osteoclasts, which is augmented or completed by additional factors such as 1,25(OH)2D3.

The role of growth factors in haemopoietic development: clinical and biological implications

Mature blood cells of all lineages are derived from a single class of cell, the haemopoietic stem cell. Stem cells are pluripotent and capable of almost limitless self-renewal. In the bone marrow they form part of a hierarchy that includes progenitor cells, which are more restricted in the lineages their progeny can adopt, and precursor cells, which are committed to differentiation. The mechanisms that regulate progression through this hierarchy are not fully understood, but evidence suggests that both bone marrow stromal cells and soluble growth factors have a role in controlling haemopoiesis. Four growth factors act on progenitor cells to promote their survival, proliferation, differentiation, and maturation: interleukin-3 (IL-3), granulocyte/macrophage-colony stimulating factor (GM-CSF), granulocyte-CSF (G-CSF), and macrophage-CSF (M-CSF). They can also activate the function of mature cells. Considerable overlap is found in the target cells for these four growth factors. We have found that growth factors acting in synergy can recruit more primitive cells than had previously been appreciated. These factors can also determine the lineage that the progeny of multipotential progenitors will adopt. Thus, colony-stimulating factors (CSFs) have the potential to regulate the development of primitive haemopoietic cells in vivo. The properties of CSFs have made them useful in treating malignant disease: G-CSF, in particular, has been used to reduce the period of neutropaenia that follows cytotoxic therapy for various malignancies. The success of these early trials gives ground for cautious optimism about the clinical use of these compounds.