Granulocyte colony-stimulating factor: structure, function and physiology

G-CSF, the guardian of granulopoiesis

A considerable amount of continuous proliferation and differentiation is required to produce daily a billion new neutrophils in an adult human. Of the few cytokines and factors known to control neutrophil production, G-CSF is the guardian of granulopoiesis. G-CSF/CSF3R signaling involves the recruitment of non-receptor protein tyrosine kinases and their dependent signaling pathways of serine/threonine kinases, tyrosine phosphatases, and lipid second messengers. These pathways converge to activate the families of STAT and C/EBP transcription factors. CSF3R mutations are associated with human disorders of neutrophil production, including severe congenital neutropenia, neutrophilia, and myeloid malignancies. More than three decades after their identification, cloning, and characterization of G-CSF and G-CSF receptor, fundamental questions remain about their physiology.

Evaluation of electrochemiluminescent technology for inhibitors of granulocyte colony-stimulating factor receptor binding

An electrochemiluminescent (ECL) assay was developed to identify compounds that inhibit the interaction of granulocyte colony-stimulating factor (GCSF) with its recombinant human receptor. The ECL technology uses a tris-(bipyridine) chelate of ruthenium, which, in the presence of excess tripropylamine, undergoes a redox reaction cycle to produce light. Paramagnetic beads with primary antibody were coated with secondary anti-GCSF receptor antibody, which were then bound with GCSF receptor. These samples were incubated with ruthenylated GCSF in the presence and absence of test compounds. The bead density, receptor and ligand concentrations, and incubation time were optimized in the assay. A set of mixed compound plates was screened to examine the feasibility of using this technology in high throughput screening. The results from this format were found to be comparable to the assay performed using a time-resolved fluorescence format.

Dexamethasone and interleukin-1 potently synergize to stimulate the production of granulocyte colony-stimulating factor in differentiated THP-1 cells

The human monocytic leukemic cell line, THP-1, which differentiates toward macrophages in response to phorbol 12-myristate 13-acetate (PMA) was investigated for its ability to produce granulocyte colony-stimulating factor (G-CSF). G-CSF protein was neither produced during PMA-induced differentiation nor in response to dexamethasone (Dex) alone. However, when combined, PMA and Dex synergistically stimulated THP-1 cells to produce G-CSF. The synergistic interaction between PMA and Dex on G-CSF production appeared to be mediated through the production of interleukin-1 (IL-1) since neutralization of IL-1 activity completely inhibited G-CSF production. Further experiments demonstrated that in THP-1 cells pretreated with PMA, Dex potently synergized with IL-1 to stimulate G-CSF production.

Characterization of the receptor binding determinants of granulocyte colony stimulating factor

We performed a series of experiments using alanine-scanning mutagenesis to locate side chains within human granulocyte colony-stimulating factor (G-CSF) that are involved in human G-CSF receptor binding. We constructed a panel of 28 alanine mutants that examined all surface exposed residues on helices A and D, as well as all charged residues on the surface of G-CSF. The G-CSF mutants were expressed in a transiently transfected mammalian cell line and quantitated by a sensitive biosensor method. We measured the activity of mutant proteins using an in vitro proliferation assay and an ELISA binding competition assay. These studies show that there is a region of five charged residues on helices A and C employed by G-CSF in binding its receptor, with the most important residue in this binding patch being Glu 19. Both wild-type G-CSF and the E19A mutant were expressed in E. coli. The re-folded proteins were found to have proliferative activities similar to the analogous proteins from mammalian cells: furthermore, biophysical analysis indicated that the E19A mutation does not cause gross structural perturbations in G-CSF. Although G-CSF is likely to signal through receptor homo-dimerization, we found no compelling evidence for a second receptor binding region. We also found no evidence of self-antagonism at high G-CSF concentrations, suggesting that, in contrast to human growth hormone (hGH) and erythropoietin (EPO), G-CSF probably does not signal via a pure 2:1 receptor ligand complex. Thus, G-CSF, while having a similar tertiary structure to hGH and EPO, uses different areas of the four helix bundle for high-affinity interaction with its receptor.

Differential alterations in plasma colony-stimulating factor concentrations in meningococcaemia

To determine whether circulating levels of any of the colony-stimulating factors (CSF) might contribute to the host response in severe sepsis, plasma concentrations of granulocyte CSF (G-CSF), granulocyte-macrophage CSF (GM-CSF), and macrophage CSF (M-CSF) were measured by immunoassays in 20 subjects with meningococcaemia, a bloodstream infection caused by Neisseria meningitidis, that has proven to be a valuable model to study the responses of other inflammatory mediators during sepsis and septic shock in humans. Plasma G-CSF concentrations were transiently elevated in most subjects during the early phase of meningococcaemia, and were higher in subjects with septic shock (mean +/- s.d. = 165 +/- 142 ng/ml, n = 9) compared with those who remained normotensive (mean +/- s.d. = 7 +/- 2 ng/ml, n = 10) (P < 0.05). Peak plasma G-CSF concentrations > 10 ng/ml were associated with the development of septic shock (P < 0.01), disseminated intravascular coagulation (P < 0.01), fulminant infection (P < 0.05), and a fatal outcome (P < 0.01). Plasma GM-CSF concentrations > 1 ng/ml were briefly present in subjects with life-threatening septic shock (1-15 ng/ml, n = 5), and were strongly associated with fulminant meningococcaemia (P < 0.01). Plasma M-CSF concentrations were marginally elevated in all subjects, but were not associated with complications related to or arising from sepsis-induced organ injury. This study demonstrates that plasma levels of G-CSF, GM-CSF and M-CSF show very different responses during meningococcaemia, changes which presumably reflect the different roles played by these mediators in sepsis and, potentially, in septic shock.

Mapping of the antibody- and receptor-binding domains of granulocyte colony-stimulating factor using an optical biosensor. Comparison with enzyme-linked immunosorbent assay competition studies

An automated optical biosensor instrument for measuring molecular interactions (Pharmacia BIAcore) has been used to characterise the epitopes recognised by 15 monoclonal antibodies raised against recombinant human granulocyte colony-stimulating factor (G-CSF). The BIAcore combines an autosampler and integrated microfluidic cartridge for the introduction and transportation of samples to the sensor chip surface, with surface plasmon resonance to detect binding events. A rabbit anti-mouse Fc antibody, coupled to the sensor surface in situ using conventional protein chemistry techniques, was used to capture an anti-G-CSF monoclonal antibody. G-CSF was bound to this antibody by injection over the sensor surface. Multi-site binding experiments were then performed in which other anti-G-CSF monoclonal antibodies were injected sequentially over the surface, and their ability to bind to the G-CSF in a multimolecular complex monitored in real time. Results obtained using the biosensor have been compared with data obtained by cross competition studies using biotinylated antibodies or antibody binding studies using chemically or enzymatically derived G-CSF peptide fragments or synthetic peptides. The results of these studies are in excellent agreement with the data from the BIAcore, although modification of the antibody or G-CSF occasionally altered the epitope affinity.