Cellular processing of murine colony-stimulating factor (Multi-CSF, GM-CSF, G-CSF) receptors by normal hemopoietic cells and cell lines

COVID-19 virtual patient cohort suggests immune mechanisms driving disease outcomes

To understand the diversity of immune responses to SARS-CoV-2 and distinguish features that predispose individuals to severe COVID-19, we developed a mechanistic, within-host mathematical model and virtual patient cohort. Our results suggest that virtual patients with low production rates of infected cell derived IFN subsequently experienced highly inflammatory disease phenotypes, compared to those with early and robust IFN responses. In these in silico patients, the maximum concentration of IL-6 was also a major predictor of CD8+ T cell depletion. Our analyses predicted that individuals with severe COVID-19 also have accelerated monocyte-to-macrophage differentiation mediated by increased IL-6 and reduced type I IFN signalling. Together, these findings suggest biomarkers driving the development of severe COVID-19 and support early interventions aimed at reducing inflammation.

Neutrophil GM-CSF receptor dynamics in acute lung injury

GM‐CSF is important in regulating acute, persistent neutrophilic inflammation in certain settings, including lung injury. Ligand binding induces rapid internalization of the GM‐CSF receptor (GM‐CSFRα) complex, a process essential for signaling. Whereas GM‐CSF controls many aspects of neutrophil biology, regulation of GM‐CSFRα expression is poorly understood, particularly the role of GM‐CSFRα in ligand clearance and whether signaling is sustained despite major down‐regulation of GM‐CSFRα surface expression. We established a quantitative assay of GM‐CSFRα surface expression and used this, together with selective anti‐GM‐CSFR antibodies, to define GM‐CSFRα kinetics in human neutrophils, and in murine blood and alveolar neutrophils in a lung injury model. Despite rapid sustained ligand‐induced GM‐CSFRα loss from the neutrophil surface, which persisted even following ligand removal, pro‐survival effects of GM‐CSF required ongoing ligand‐receptor interaction. Neutrophils recruited to the lungs following LPS challenge showed initially high mGM‐CSFRα expression, which along with mGM‐CSFRβ declined over 24 hr; this was associated with a transient increase in bronchoalveolar lavage fluid (BALF) mGM‐CSF concentration. Treating mice in an LPS challenge model with CAM‐3003, an anti‐mGM‐CSFRα mAb, inhibited inflammatory cell influx into the lung and maintained the level of BALF mGM‐CSF. Consistent with neutrophil consumption of GM‐CSF, human neutrophils depleted exogenous GM‐CSF, independent of protease activity. These data show that loss of membrane GM‐CSFRα following GM‐CSF exposure does not preclude sustained GM‐CSF/GM‐CSFRα signaling and that this receptor plays a key role in ligand clearance. Hence neutrophilic activation via GM‐CSFR may play an important role in neutrophilic lung inflammation even in the absence of high GM‐CSF levels or GM‐CSFRα expression.

Growth and Differentiation Factors

This review is restricted to neutrophilic granulocytes (granulocytes), monocytes (macrophages), and eosinophils, with only passing reference to cells that are also usually included in the "myeloid" category-megakaryocytes, mast cells, and erythroid cells. Although some dendritic cells are of myeloid origin, they are discussed elsewhere. The validity of the information to be described depends on two assumptions: (a) that in vitro data are applicable to events in vivo and (b) that mouse data reflect events in man. Both assumptions are likely to be broadly correct.

Protein engineering and preclinical development of a GM-CSF receptor antibody for the treatment of rheumatoid arthritis

BACKGROUND AND PURPOSE

For antibody therapies against receptor targets, in vivo outcomes can be difficult to predict because of target-mediated clearance or antigen 'sink' effects. The purpose of this work was to engineer an antibody to the GM-CSF receptor α (GM-CSFRα) with pharmacological properties optimized for chronic, s.c. treatment of rheumatoid arthritis (RA) patients.

EXPERIMENTAL APPROACH

We used an in silico model of receptor occupancy to guide the target affinity and a combinatorial phage display approach for affinity maturation. Mechanism of action and internalization assays were performed on the optimized antibody in vitro before refining the modelling predictions of the eventual dosing in man. Finally, in vivo pharmacology studies in cynomolgus monkeys were carried out to inform the predictions and support future clinical development.

KEY RESULTS

Antibody potency was improved 8600-fold, and the target affinity was reached. The refined model predicted pharmacodynamic effects at doses as low as 1 mg kg(-1) and a study in cynomolgus monkeys confirmed in vivo efficacy at 1 mg kg(-1) dosing.

CONCLUSIONS AND IMPLICATIONS

This rational approach to antibody drug discovery enabled the isolation of a potent molecule compatible with chronic, s.c. self-administration by RA patients. We believe this general approach enables the development of optimal biopharmaceuticals.

Pegylated filgrastim is comparable with filgrastim as support for commonly used chemotherapy regimens: a multicenter, randomized, crossover phase 3 study

The purpose of this study was to compare the efficacy and safety of a single subcutaneous injection of pegylated filgrastim with daily filgrastim as a prophylaxis for neutropenia induced by commonly used chemotherapy regimens. Fifteen centers enrolled 337 chemotherapy-naive cancer patients with normal bone marrow function. All patients randomized into AOB and BOA arms received two cycles of chemotherapy. Patients received a single dose of pegylated filgrastim 100 µg/kg in cycle 1 (AOB) or cycle 2 (BOA) and daily doses of filgrastim 5 µg/kg/day in cycle 1 (BOA) or cycle 2 (AOB). Efficacy and safety parameters were recorded. The primary end point was the rate of protection against grade 4 neutropenia after chemotherapy [defined as the rate at which the absolute neutrophil count (ANC) remained >0.5×10(9)/l throughout the entire cycle]. Ninety-four percent of patients receiving pegylated filgrastim or filgrastim did not develop grade 4 neutropenia. The incidence of ANC<1.0×10(9)/l was 16.0% (50/313) after support with either pegylated filgrastim or filgrastim. The incidences of febrile neutropenia and antibiotic administration were similar in both groups. Notably, faster ANC recovery was observed with pegylated filgrastim support. The ANC nadir was also earlier with pegylated filgrastim (day 7) support than with filgrastim support (day 9), although the depth of nadir was not significantly different. A single subcutaneous injection of pegylated filgrastim 100 μg/kg provided adequate and safe neutrophil support comparable with daily subcutaneous injections of unmodified filgrastim 5 μg/kg/day in patients receiving commonly used standard-dose mild-to-moderate myelosuppressive chemotherapy regimens.

Analysis of cytokines and chemokines produced by whole blood, peripheral mononuclear and polymorphonuclear cells

Cytokines are immunomodulating proteins involved in cellular communication. The levels of different cytokines reflect the immune capabilities of a person. In literature both whole blood and peripheral blood mononuclear cells (PBMCs) are used, which might lead to different results. The choice between these different sources is not always explained. The goal of our experiments is to determine the cytokine response of whole blood, PBMCs and polymorphonuclear cells (PMNs) after stimulation with lipopolysaccharide (LPS). We used a multiplex analysis to determine a difference in cytokine secretion patterns. In general, PBMCs demonstrated the highest cytokine production and PMNs have an overall low cytokine production. CCL11 and interleukin-23 (IL-23) (and IL-12p40) were exclusively expressed in whole blood. IL-20, VEGF and GM-CSF were expressed only by PBMCs. This difference in expression could be explained by the bioactive components in serum, presence and interaction with granulocytes or platelets in whole blood, the anticoagulant heparin in whole blood and others. The expression of cytokines by cells is dependent on the microenvironment. Different conditions lead to different results. We recommend a thorough examination of the conditions before performing experiments.

Mechanistic modeling of antigen sink effect for mavrilimumab following intravenous administration in patients with rheumatoid arthritis

Mavrilimumab is a fully human monoclonal antibody that binds to granulocyte-macrophage colony stimulating factor receptor α (GM-CSFRα) with high affinity and specificity and has potential application in various inflammatory diseases. The objective of this investigation was to develop a mechanistic population model to characterize the pharmacokinetics of mavrilimumab, the GM-CSFRα-mediated clearance, and receptor occupancy following single intravenous dosing to patients with rheumatoid arthritis. The internalization rate of mavrilimumab-GM-CSFRα complex was fixed to a value determined from quantitative confocal fluorescent imaging. The estimated typical first-order clearance and the central and peripheral distribution volumes were 3.79 mL/kg/d, 39.6 mL/kg, and 50.3 mL/kg, respectively. The systemic GM-CSFRα expression level was estimated to be 0.0782 nM, and the equilibrium dissociation constant (0.103 nM) was in good agreement with the monovalent affinity determined by surface plasmon resonance. By fitting to the observed pharmacokinetic data, the mechanistic model predicted that systemically greater than 90% GM-CSFRα blockade by mavrilimumab was achieved and maintained up to 4, 7, and 11 weeks following single 1-, 3-, and 10-mg/kg administrations, respectively. Posterior visual predictive check and bootstrapping suggest that the mechanistic model is reasonably robust and can be used to predict mavrilimumab exposure under various scenarios for future clinical trial design.

Characteristics of soluble and membrane-bound forms of haemopoietic growth factor receptors: relationships to biological function

In order to understand how extracellular growth factors (colony-stimulating factors, CSFs) induce biological responses in haemopoietic cells it is necessary first to describe the primary binding characteristics of isolated receptors and then to determine how living cells modify these binding characteristics and process the ligand-receptor complexes. We have solubilized multi-CSF (interleukin 3) and granulocyte macrophage CSF (GM-CSF) receptors in 1% Triton X-100 and developed methods to study their binding characteristics in solution. Whilst the multi-CSF receptor exhibits the same binding characteristics in solution and in the cellular state, the GM-CSF receptor appears to be converted to a lower affinity form in solution. In intact cells at biological temperatures both types of receptor and ligand are internalized and degraded under steady-state conditions; the kinetic processes underlying the maintenance of the steady state are characteristic for each type of receptor and each type of cell. We have determined the kinetic constants for a variety of CSFs and cell types and correlated steady-state receptor occupancy with biological responses. The importance of CSF utilization rate by responsive cells has also been evaluated as a mechanism for limiting biological responses.

Suppressor of cytokine signaling (SOCS)-5 is a potential negative regulator of epidermal growth factor signaling

The suppressors of cytokine signaling (SOCS) proteins are a family of SH2 domain-containing intracellular inhibitors of cytokine signal transduction that act by several different mechanisms. Recent evidence suggests that the action of the SOCS proteins may extend beyond the cytokine receptors to signaling initiated by members of the tyrosine kinase receptor family. In this study, the ability of SOCS-5 to negatively regulate signaling cascades downstream of the epidermal growth factor receptor (EGF-R) has been examined by using an EGF-responsive cell line engineered to constitutively express the EGF-R and SOCS-5 or SOCS-5 mutants. SOCS-5 associated with the EGF-R complex in an EGF-independent manner, and the mitogenic response to EGF of all SOCS-5-expressing cell lines was dramatically inhibited when compared with control cell lines. Furthermore, this effect was abrogated after deletion of the SOCS-5 SOCS box. This result suggests that the inhibition of signaling occurs through enhanced proteasomal degradation of the EGF-R through SOCS box recruitment of E3 ubiquitin ligase activity.

GMCSF activates NF-kappaB via direct interaction of the GMCSF receptor with IkappaB kinase beta

Granulocyte-macrophage colony-stimulating factor (GMCSF) has a central role in proliferation and differentiation of hematopoetic cells. Furthermore, it influences the proliferation and migration of endothelial cells. GMCSF elicits these functions by activating a receptor consisting of a ligand-specific alpha-chain and a beta-chain, which is common for GMCSF, interleukin-3 (IL-3), and IL-5. It is known that various signaling molecules such as Janus kinase 2 or transcription factors of the signal transducer and activator of transcription (STAT) family bind to the common beta-chain and initiate signaling cascades. However, alpha-chain-specific signal transduction adapters have to be postulated given that IL-3, IL-5, and GMCSF induce partly distinct biologic responses. Using a yeast 2-hybrid system, we identified the alpha-chain of the GMCSF receptor (GMRalpha) as putative interaction partner of IkappaB kinase beta, one of the central signaling kinases activating the transcription factor nuclear factor-kappaB (NF-kappaB). Using endogenous protein levels of endothelial cell extracts, we could verify the interaction by coimmunoprecipitation experiments. Fluorescence resonance energy transfer (FRET) microscopy confirmed the direct interaction of CFP-IKKbeta and YFPGMRalpha in living cells. Functional studies demonstrated GMCSF-dependent activation of IkappaB kinase activity in endothelial cells, degradation of IkappaB, and activation of NF-kappaB. Further biologic studies using GMCSF-dependent TF-1 cells indicated that GMCSF-triggered activation of NF-kappaB is important for cell survival and proliferation.

High affinity interleukin-3 receptor expression on blasts from patients with acute myelogenous leukemia correlates with cytotoxicity of a diphtheria toxin/IL-3 fusion protein

Diphtheria fusion proteins are a novel class of agents for the treatment of chemotherapy resistant AML. We prepared DT(388)IL3 composed of human interleukin-3 (IL3) fused to the catalytic and translocation domain of diphtheria toxin (DT(388)) and assessed its activity on patient AML blasts. The number and affinity of IL3 receptors in circulating blasts was measured using a radiolabeled IL3 agonist (SC-65461). Ninety-two percent of patients' blasts had both high and low affinity IL3 receptors. DT(388)IL3 cytotoxicity (>1 log cell kill) was seen in nine of 25 samples (36%). There was a significant correlation between DT(388)IL3 log cell kill and blast high affinity IL3 receptor density (P=0.0044). These results show that specific high affinity IL3 binding is one factor important in the sensitivity of patients' leukemic blasts to DT(388)IL3.

Characterization of diphtheria fusion proteins targeted to the human interleukin-3 receptor

Diphtheria fusion proteins are chimeric proteins consisting of the catalytic and translocation domains of diphtheria toxin (DT(388)) linked through an amide bond to one of a variety of peptide ligands. The ligand targets the molecule to cells and the toxin enters the cell, inactivates protein synthesis and induces cell death. Diphtheria fusion proteins directed to human myeloid leukemic blasts are a novel class of therapeutics for patients with chemotherapy refractory myeloid leukemia. Because of the presence of interleukin-3 (IL3) receptors on myeloid leukemic progenitors and its absence from mature myeloid cells, we synthesized four bacterial expression vectors encoding DT(388) fused to human IL3. Different molecules were engineered to assess the effects of modifications on yield, purity and potency of product. The constructs differed in the size of the linker peptide between the DT(388) and IL3 domains and in the presence or absence of an oligohistidine tag on the N- or C-terminus. Escherichia coli were transformed and recombinant protein induced and purified from inclusion bodies. Similar final yields of 3-6 mg of purified protein per liter of bacterial culture were obtained with each of the four molecules. Purity ranged from 70 to 90% after partial purification by anion-exchange, size-exclusion chromatography and/or nickel affinity chromatography. Proteins were soluble and stable at 4 degrees C and -80 degrees C in phosphate-buffered saline at 0.03-0.5 mg/ml. The fusion proteins showed predicted molecular weights by SDS-PAGE, HPLC and tandem mass spectrometry and had full ADP-ribosylating activities. Each was immunoreactive with antibodies to DT(388) and IL3. Each of the fusion proteins with the exception of the one with an N-terminal oligohistidine tag showed full IL3 receptor binding affinity (K:(d) = 3 nM) and potent and selective cytotoxicity to IL3 receptor positive human myeloid leukemia cell lines (IC(50) = 5-10 pM). In contrast, the N-terminal histidine-tagged fusion protein bound IL3 receptor with a 10-fold lower affinity and was 10-fold less cytotoxic to IL3 receptor positive blasts. Thus, we report a series of novel, biologically active DT(388)IL3 fusion proteins for potential therapy of patients with receptor positive myeloid leukemias.

Diphtheria toxin fused to human interleukin-3 is toxic to blasts from patients with myeloid leukemias

Leukemic blasts from patients with acute phase chronic myeloid leukemic and refractory acute myeloid leukemia are highly resistant to a number of cytotoxic drugs. To overcome multi-drug resistance, we engineered a diphtheria fusion protein by fusing human interleukin-3 (IL3) to a truncated form of diphtheria toxin (DT) with a (G4S)2 linker (L), expressed and purified the recombinant protein, and tested the cytotoxicity of the DTLIL3 molecule on human leukemias and normal progenitors. The DTLIL3 construct was more cytotoxic to interleukin-3 receptor (IL3R) bearing human myeloid leukemia cell lines than receptor-negative cell lines based on assays of cytotoxicity using thymidine incorporation, growth in semi-solid medium and induction of apoptosis. Exposure of mononuclear cells to 680 pM DTLIL3 for 48 h in culture reduced the number of cells capable of forming colonies in semi-solid medium (colony-forming units leukemia) > or =10-fold in 4/11 (36%) patients with myeloid acute phase chronic myeloid leukemia (CML) and 3/9 (33%) patients with acute myeloid leukemia (AML). Normal myeloid progenitors (colony-forming unit granulocyte-macrophage) from five different donors treated and assayed under identical conditions showed intermediate sensitivity with three- to five-fold reductions in colonies. The sensitivity to DTLIL3 of leukemic progenitors from a number of acute phase CML patients suggests that this agent could have therapeutic potential for some patients with this disease.

Granulocyte colony-stimulating factor serum and urine concentrations in neutropenic neonates before and after intravenous administration of recombinant granulocyte colony-stimulating factor

BACKGROUND

Recombinant granulocyte colony-stimulating factor (rG-CSF) has been suggested as a treatment for certain varieties of neonatal neutropenia, but little is known about the pharmacologic disposition of rG-CSF in that population.

METHODS

Ten neutropenic neonates were treated with rG-CSF, 10 micrograms/kg intravenously once daily for 3 to 5 days. Serum and urine samples were obtained before rG-CSF dosing and at intervals thereafter for G-CSF quantification by enzyme-linked immunosorbent assay.

RESULTS

Five of the neutropenic neonates (termed group 1) were not infected but likely had hyporegenerative neutropenia (4 were born after pregnancy-induced hypertension/intrauterine growth restriction, and 1 had Rh hemolytic disease). Five other infants (group 2) had neutropenia accompanying bacterial sepsis and shock. Before receiving the first dose of rG-CSF, endogenous G-CSF serum and urine concentrations were relatively low in group 1, averaging 130 pg/mL (range: 48-209) in serum and 53 pg/mL (range: 15-141) in urine. Serum concentrations immediately before the final dose were much higher (range: 81-24 835 pg/mL), whereas urine concentrations were unchanged (range: <7 pg/mL-126 pg/mL). In group 2 patients, before receiving the first-dose of rG-CSF, endogenous concentrations were very high, averaging 59 575 pg/mL (range: 20 028-98 280) in serum and 3189 pg/mL (range: 23-4770) in urine. Predose serum concentrations before the final dose (range: 427-14 460 pg/mL) were lower than before the first dose. The area under the concentration curve after the first dose of rG-CSF administration in group 1 was significantly lower than after the first dose in group 2, but no difference in area under the concentration curve was observed between groups 1 and 2 after the last dose of rG-CSF.

SPECULATION

The principal means of clearing G-CSF from the serum is by saturable binding to specific G-CSF receptors (G-CSF-Rs). Therefore, the very high G-CSF serum and urine concentrations of group 2 patients before the first rG-CSF dose implies that their G-CSF-Rs were saturated before the dose was given. We speculate that if G-CSF-Rs are saturated with endogenous G-CSF, treatment with rG-CSF will add little or nothing to the granulocytopoietic effort. On this basis, we judge that neonates with septic shock and neutropenia are unlikely to derive benefit from rG-CSF administration.

Targeting myeloid leukemia with a DT(390)-mIL-3 fusion immunotoxin: ex vivo and in vivo studies in mice

The IL-3 receptor was expressed on a high frequency of myeloid leukemia cells and also on hematopoietic and vascular cells. We previously showed that a recombinant IL-3 fusion immunotoxin (DT(390)IL-3) expressed by splicing the murine IL-3 gene to a truncated diphtheria toxin (DT(390)) gene selectively killed IL-3R(+) expressing cells and was not uniformly toxic to uncommitted BM progenitor cells (Chan,C.-H., Blazar,B.R., Greenfield,L., Kreitman,R.J. and Vallera,D.A., 1996, Blood, 88, 1445-1456). Thus, we explored the feasibility of using DT(390)IL-3 as an anti-leukemia agent. DT(390)IL-3 was toxic when administered to mice at doses as low as 0.1 microg/day. The dose limiting toxicity appeared to be related to platelet and bleeding effects of the fusion toxin. Because of these effects, DT(390)IL-3 was studied ex vivo as a means of purging contaminating leukemia cells from BM grafts in a murine autologous BM transplantation. In this setting, as few as 1000 IL-3R-expressing, bcr/abl transformed myeloid 32Dp210 leukemia cells were lethal. An optimal purging interval of 10 nM/l for 8 h eliminated leukemia cells from 32Dp210/BM mixtures given to lethally irradiated (8 Gy) C3H/HeJ syngeneic mice. Mice given treated grafts containing BM and a lethal dose of 32Dp210 cells survived over 100 days while mice given untreated grafts did not survive (P < 0.00001). DT(390)IL-3 may prove highly useful for ex vivo purging of lethal malignant leukemia cells from autologous BM grafts.

Receptor Clearance Obscures the Magnitude of Granulocyte-Macrophage Colony-Stimulating Factor Responses in Mice to Endotoxin or Local Infections

Marrow cells from mice lacking high-affinity receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF; βc−/− mice) were shown to bind and internalize much less GM-CSF than cells from normal (βc+/+) mice. βc−/− mice were used to determine the effect of negligible receptor-mediated clearance on detectible GM-CSF responses to the intravenous injection of endotoxin or the intraperitoneal injection of casein plus microorganisms. Unlike the minor serum GM-CSF responses to endotoxin seen in βc+/+ mice, serum GM-CSF levels rose 30-fold to 9 ng/mL in βc−/− mice even though loss of GM-CSF in the urine was greater than in βc+/+ mice. Organs from βc−/− and βc+/+ mice had a similar capacity to produce GM-CSF in vitro, as did peritoneal cells from both types of mice when challenged in vitro by casein. However, when casein was injected intraperitoneally, βc−/− mice developed higher and more sustained levels of GM-CSF than did βc+/+ mice. The data indicated that receptor-dependent removal of GM-CSF masks the magnitude of GM-CSF responses to endotoxin and local infections. Because of this phenomenon, serum GM-CSF concentrations can be a misleading index of the occurrence or nonoccurrence of GM-CSF responses to infections.

Receptor Clearance Obscures the Magnitude of Granulocyte-Macrophage Colony-Stimulating Factor Responses in Mice to Endotoxin or Local Infections

Marrow cells from mice lacking high-affinity receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF; βc−/− mice) were shown to bind and internalize much less GM-CSF than cells from normal (βc+/+) mice. βc−/− mice were used to determine the effect of negligible receptor-mediated clearance on detectible GM-CSF responses to the intravenous injection of endotoxin or the intraperitoneal injection of casein plus microorganisms. Unlike the minor serum GM-CSF responses to endotoxin seen in βc+/+ mice, serum GM-CSF levels rose 30-fold to 9 ng/mL in βc−/− mice even though loss of GM-CSF in the urine was greater than in βc+/+ mice. Organs from βc−/− and βc+/+ mice had a similar capacity to produce GM-CSF in vitro, as did peritoneal cells from both types of mice when challenged in vitro by casein. However, when casein was injected intraperitoneally, βc−/− mice developed higher and more sustained levels of GM-CSF than did βc+/+ mice. The data indicated that receptor-dependent removal of GM-CSF masks the magnitude of GM-CSF responses to endotoxin and local infections. Because of this phenomenon, serum GM-CSF concentrations can be a misleading index of the occurrence or nonoccurrence of GM-CSF responses to infections.

Close association between clearance of recombinant human granulocyte colony-stimulating factor (G-CSF) and G-CSF receptor on neutrophils in cancer patients

Recombinant human granulocyte colony-stimulating factor (rhG-CSF) is used to counter chemotherapy-induced neutropenia. Our previous study showed an inverse correlation between serum rhG-CSF levels and the number of circulating neutrophils in cancer patients (H. Takatani, H. Soda, M. Fukuda, M. Watanabe, A. Kinoshita, T. Nakamura, and M. Oka, Antimicrob. Agents Chemother. 40:988-991, 1996). The aim of this study was to clarify the relationship between rhG-CSF clearance and G-CSF receptors on circulating neutrophils. In five cancer patients receiving chemotherapy, a bolus dose of rhG-CSF (5 microg/kg) was injected intravenously during defined phases of posttreatment neutropenia and neutrophilia. Serum rhG-CSF levels were measured by a chemiluminescence enzyme immunoassay and analyzed by moment analysis. G-CSF receptors on neutrophils were detected by flow cytometry with biotinylated rhG-CSF. rhG-CSF clearance was significantly higher at neutrophilia than at neutropenia (1,497 +/- 132 versus 995 +/- 266 ml/h; P < 0.01). The percentage of G-CSF receptor-positive neutrophils, reflecting the number of G-CSF receptors per cell, was low at neutropenia without rhG-CSF therapy (44.5% +/- 22.1%) and high at neutrophilia with rhG-CSF therapy (73. 0% +/- 11.4%; P < 0.01). rhG-CSF clearance closely correlated with the percentage of G-CSF receptor-positive neutrophils (r2 = 0.91; P < 0.0001) and neutrophil count (r2 = 0.72; P < 0.005). Our results indicate that, in cancer patients receiving chemotherapy, rhG-CSF increases the number of G-CSF receptors per cell as well as circulating neutrophil counts, resulting in modulation of its own clearance.

Toxicology and pharmacokinetics of DT388-GM-CSF, a fusion toxin consisting of a truncated diphtheria toxin (DT388) linked to human granulocyte-macrophage colony-stimulating factor (GM-CSF) in C57BL/6 mice

Because the majority of acute myeloid leukemia (AML) blasts express the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor, we are developing a fusion toxin consisting of a truncated diphtheria toxin (DT388) linked to human GM-CSF for multi-drug resistant AML. Our goal was to determine the toxicity and pharmacokinetics of DT388-GM-CSF in C57BL/6 mice. Because human GM-CSF does not cross-react with the mouse GM-CSF receptor, the toxicity observed should be nonspecific toxicity of DT388. We injected C57BL/6 mice i.p. with 0.1, 0.5, 1.0, 1.5, 1.75, 2.0, 3.5, 5.0, or 10 micrograms/day of DT388-GM-CSF for 5 consecutive days. For pharmacokinetics, blood samples were drawn at 20, 40, 60, 120, and 180 min after i.p. administration of 81 micrograms/kg of DT388-GM-CSF. In mice, the LD10 of DT388-GM-CSF is between 84.4 (1.5) and 104.4 (1.75) micrograms/kg (microgram) when administered for 5 consecutive days. All mice receiving > or = 201 micrograms/kg (3.5 micrograms) for 5 consecutive days died. Histopathologic examination of morbid animals showed only renal toxicity with acute proximal tubular necrosis. DT388-GM-CSF is stable in vivo based on nonreducing SDS-PAGE gel of plasma samples of 125I-labeled DT388-GM-CSF injected i.p.. The peak concentration of DT388-GM-CSF was 3.3 x 10(-8) M at 40 min and exhibited a t1/2 of 24 min. Based on its half-life, DT388-GM-CSF concentrations in the plasma are above the concentration inhibiting 50% protein synthesis and inducing apoptosis in 50% of HL-60 cells (AML cell line) for 5.2 h. Only four of 17 mice developed a weak immune response (0.9-160 ng/mL) 3 weeks after treatment. DT388-GM-CSF exhibits a short t1/2, but concentrations exceed those required in vitro to inhibit AML cell lines.

A recombinant GM-CSF-PE40 ligand toxin is functionally active but not cytotoxic to cells

A granulocyte/macrophage colony-stimulating factor (GM-CSF)-Pseudomonas exotoxin (PE) 40 fusion protein was constructed for potential use in the treatment of myeloid leukaemias, as a conditioning agent prior to allogeneic bone marrow transplantation or for ex vivo purging of malignant cells prior to autologous bone marrow transplantation. The GM-CSF-PE40 fusion protein successfully binds to the GM-CSF receptor and is capable of initiating a mitogenic signal similar to native GM-CSF in the GM-CSF-dependent TF1 cell line. The toxin component also appears to be fully functional as determined by an in vitro adenosine diphosphate-ribosylation assay. The GM-CSF-PE40 fusion protein, however, was not cytotoxic to a number of myeloid leukaemia cell lines. It is suggested that the mechanism of internalization of the GM-CSF receptor is not appropriate for the translocation of PE to the cytosol where it can fulfil its cytotoxic potential.

Pharmacokinetics and pharmacodynamics of a recombinant human granulocyte colony-stimulating factor

Granulocyte colony-stimulating factor (G-CSF), a hematopoietic growth factor, is a clinically effective drug used to promote neutrophil recovery in patients with chemo- or radiotherapy-induced neutropenia. We have reviewed the pharmacokinetic and pharmacodynamic properties of three kinds of G-CSFs: E. coli derived G-CSF, CHO-derived G-CSF, and mutein G-CSF. The clearances of G-CSFs are saturable and autoinducible in experimental animals and humans. That is, the systemic clearances of G-CSFs decrease as the dose injected increases and approaches a constant value. Both saturable and nonsaturable processes are involved in G-CSF elimination. Also, the systemic clearances of G-CSFs are increased by repeated administration of G-CSF. Although the relative bioavailability of G-CSFs after subcutaneous administration is approximately 60%, the increase in peripheral white blood cells or neutrophils is greater than that after intravenous administration at the same dose. The effects of G-CSFs seem to be time dependent rather than AUC dependent, considering that mean residence time of G-CSFs in the plasma is longer after subcutaneous administration than that after intravenous administration. There is a slight difference in the pharmacokinetics of E-coli- and CHO-G-CSF although they seem to be pharmacologically equivalent. The correlation between G-CSF clearance and peripheral neutrophil counts in the patients suggests that G-CSF receptors contribute to G-CSF clearance. Quantitative pharmacokinetic analysis using mutein G-CSF shows that the G-CSF receptor plays a major role in saturable G-CSF clearance, and that this saturable process accounts for approximately 80% of the total clearance at low doses. That is, the degradation following the receptor-mediated endocytosis in bone marrow might be a major clearance system of G-CSF at a physiological blood level. The G-CSF receptor in bone marrow might work not only as a signal transducer for differentiation and proliferation of granulopoietic precurcer cells but as a regulator of G-CSF levels in blood. In addition, at high doses, glomerular filtration in the kidneys is the major process for nonsaturable G-CSF clearance. At present, polyethylene glycol derivatives of G-CSF are being developed to reduce the frequency of G-CSF administration.

Kinetic analysis of receptor-mediated endocytosis of G-CSF derivative, nartograstim, in rat bone marrow cells

To elucidate the mechanism of the receptor-mediated clearance of granulocyte colony-stimulating factor (G-CSF), we performed kinetic analyses of the receptor-mediated endocytosis (RME) processes using a human G-CSF derivative, nartograstim (NTG), and isolated rat bone marrow cells. The first-order rate constants involved in RME processes were obtained by computerized model fitting of the time courses of the ligand-receptor complex on both the cell surface and in the cell interior and the degradation products in the medium in the pulse-chase experiment. They were also calculated based on a kinetic model involving the ligand concentration dependence of the initial binding rate, the steady-state degradation rate, and the steady-state amounts of ligand on both the cell surface and in the interior. The rate constants for the RME processes after receptor binding determined in the different experiments were similar, that is, the half-times for the dissociation, internalization, and degradation of the ligand-receptor complex were 770, 10-30, and 20 min, respectively. However, the association constant obtained by measuring the initial binding was fivefold greater than that calculated under steady-state conditions. These kinetic analyses support the hypothesis that the internalization of the receptor may be accelerated by ligand binding, causing downregulation of the receptor on the cell surface. These overall kinetic analyses based on steady-state and non-steady-state data of the RME processes clarify the dynamics of the interaction between NTG and its receptor.

Levels of recombinant human granulocyte colony-stimulating factor in serum are inversely correlated with circulating neutrophil counts

Recombinant human granulocyte colony-stimulating factor (rhG-CSF) is effective in countering chemotherapy-induced neutropenia. However, serum rhG-CSF levels cannot be maintained throughout the course of rhG-CSF therapy. The drop in serum rhG-CSF levels may vary with the duration of rhG-CSF administration or with the circulating neutrophil counts. We investigated the relationship between serum G-CSF levels and circulating neutrophil counts and the pharmacokinetics of rhG-CSF for patients with lung cancer who had been treated with myelosuppressive chemotherapy and then with subcutaneous rhG-CSF (lenograstim, 2 micrograms per kg of body weight per day). Twelve patients were randomly assigned to four groups with different rhG-CSF therapy schedules. Serum G-CSF levels were measured by an enzyme immunoassay method. Serum G-CSF levels during the rhG-CSF therapy greatly exceeded endogenous G-CSF levels and were mainly due to the presence of exogenous rhG-CSF rather than increased levels of endogenous G-CSF. Despite the duration of rhG-CSF administration, serum G-CSF levels during rhG-CSF therapy were inversely correlated with circulating neutrophil counts (r2 = 0.73, P < 0.0001). The value for the area under the concentration-time curve of rhG-CSF on the day of neutrophilia was lower than that on the day of neutropenia (P < 0.05). Our results suggest that the fall in serum G-CSF levels during rhG-CSF therapy may result from increased clearance and/or decreased absorption of rhG-CSF, two processes related to circulating neutrophil counts.

Differential expression of granulocyte-macrophage colony-stimulating factor and IL-3 receptor subunits on human CD34+ cells and leukemic cell lines

Cytokines transduce their signals through specific receptors. Receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-3, and IL-5 share the common signal transducing subunit (beta c), whereas the alpha subunits function as specific ligand binding components. In this study we prepared specific mouse monoclonal antibodies against human GM-CSF receptor-alpha subunit (hGMR alpha) by immunizing mice with Ba/F3 cells transfected with hGMR alpha complementary DNA. Using these anti-hGMR alpha antibodies in combination with antibodies against IL-3 receptor-alpha (IL-3R alpha), beta c subunits, and c-kit, we examined expression patterns and modulation of these receptor subunits on several human hematopoietic cells, including CD34+ cells and leukemic cell lines. GMR alpha and IL-3R alpha were expressed on GM-CSF- and IL-3-responsive cell lines, such as TF-1 and UT-7, whereas the expression levels were much lower on UT-7E, a GM-CSF- and IL-3-unresponsive subline of UT-7. The GMR alpha subunit was expressed only on mature granulocytes and monocytes, and IL-3R alpha was expressed on monocytes but not on mature granulocytes, and none of these subunits were expressed on lymphocytes. For CD34+ cells, GMR alpha was expressed more abundantly on CD34+ CD33high cells than on CD34+ CD33low cells, whereas IL-3R alpha was expressed more abundantly on CD34+ CD33low cells than on CD34+ CD33high and CD34+ CD33neg cells. Slight but significant expression of the beta c subunit was detected on CD34+ cells. Expression of not only GMR alpha and IL-3R alpha subunits but also c-kit was specifically downregulated by 48-hour incubation with their respective ligands. Receptor transmodulation between GM-CSF, IL-3, and stem cell factor (or kit ligand) was not detected on CD34+ cells in 48-hour cultures. We also detected upregulation of these alpha subunits by IL-1 alpha and interferon-gamma on leukemic cell lines. Our study showed expression levels for each receptor subunit--including GMR, IL-3R, and c-kit on human bone marrow and peripheral blood cells and leukemic cell lines--and revealed differential regulation of the expression of the receptor subunits.

Biotinylation of interleukin-2 (IL-2) for flow cytometric analysis of IL-2 receptor expression. Comparison of different methods

The main prerequisites for the use of biotinylated ligands to study the expression of growth factor receptors on heterogeneous cell populations, such as peripheral blood or bone marrow, by flow cytometric methods, are that the biotinylated ligand retains its binding ability and that binding of the biotinylated ligand to the receptor does not inhibit the subsequent interaction of biotin with fluorescently tagged avidin or streptavidin. Using interleukin-2 (IL-2), we compared the usefulness of various biotinylation reagents, NHS-biotin, S-NHS-biotin, S-NHS-LC-biotin, DBB and photobiotin, and developed optimal biotinylation conditions for the preparation of biologically active biotin-labeled IL-2 and the detection of IL-2 receptor expressing cells by flow cytometry. As determined by spot blot analysis, biotinylation of IL-2 was most efficient at the highest biotin-to-protein (B:P) ratio used. At a B:P ratio of 100, most of the biological activity of IL-2 was retained when S-NHS-LC-biotin was used. In contrast, most of the biological activity of IL-2 samples that were labeled with NHS-biotin or photobiotin was lost under these conditions. Biotin-labeled IL-2 preparations were tested in order to detect IL-2 receptors on IL-2 dependent CTLL-2 cells by flow cytometry after sequential staining with the biotinylated IL-2 and fluorescence tagged streptavidin. A high B:P ratio generally resulted in a high specific fluorescence intensity of the cells, particularly when S-NHS-LC-biotin was used as the biotinylation reagent. Biotin-IL-2 could also be used to detect IL-2 receptors expressed by lymphocytes in peripheral blood and bone marrow. Comparison of staining of lymphocytes with biotinylated IL-2 and an antibody against the IL-2 receptor alpha chain demonstrated that only a subset of the cells that showed a strong fluorescence signal after staining with biotinylated IL-2 expressed high numbers of the IL-2 receptor alpha chain. This is in agreement with the expression of functional IL-2 receptors on resting T cells and NK cells which do not express the alpha chain. After stimulation with PHA, virtually all lymphocytes expressed the alpha chain, whereas only part of these cells showed a strong fluorescence signal after staining with biotin-IL-2, while the rest of the cells had very low numbers of IL-2 binding sites. Our results demonstrate that, in addition to staining individual receptor subunits with antibodies, staining with biotinylated IL-2 is a useful indicator of functional IL-2 receptor expression.

Specific human granulocyte-macrophage colony-stimulating factor antagonists

Human granulocyte-macrophage colony-stimulating factor (GM-CSF) is a pleiotropic hemopoietic growth factor and activator of mature myeloid cell function. We have previously shown that residue 21 in the first helix of GM-CSF plays a critical role in both biological activity and high-affinity receptor binding. We have now generated analogues of GM-CSF mutated at residue 21, expressed them in Escherichia coli, and examined them for binding, agonistic, and antagonistic activities. Binding experiments showed that GM E21A, E21Q, E21F, E21H, E21R, and E21K bound to the GM-CSF receptor alpha chain with a similar affinity to wild-type GM-CSF and had lost high-affinity binding to the GM-CSF receptor alpha-chain-common beta-chain complex. From these mutants, only the charge reversal mutants E21R and E21K were completely devoid of agonistic activity. Significantly we found that E21R and E21K antagonized the proliferative effect of GM-CSF on the erythroleukemic cell line TF-1 and primary acute myeloid leukemias, as well as GM-CSF-mediated stimulation of neutrophil superoxide production. This antagonism was specific for GM-CSF in that no antagonism of interleukin 3-mediated TF-1 cell proliferation or tumor necrosis factor alpha-mediated stimulation of neutrophil superoxide production was observed. E. coli-derived GM E21R and E21K were effective antagonists of both nonglycosylated and glycosylated wild-type GM-CSF. These results show that low-affinity GM-CSF binding can be dissociated from receptor activation and have potential clinical significance for the management of inflammatory diseases and certain leukemias where GM-CSF plays a pathogenic role.

Cytotoxicity of a recombinant diphtheria toxin-granulocyte colony-stimulating factor fusion protein on human leukemic blast cells

Granulocyte colony-stimulating factor (G-CSF) is a potent stimulator of the growth of normal and malignant hematopoietic cells and synergizes with other factors such as interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The action of G-CSF is mediated through a specific membrane receptor, however it is not clear if all of the effects of G-CSF are direct or indirect. As a step towards addressing this problem, a recombinant diphtheria toxin (DT)-related human G-CSF fusion protein has been constructed and purified from E. coli. The 70,000 dalton chimeric protein has immunologic determinants characteristic of both DT and G-CSF. At high concentrations, DAB486-G-CSF is cytotoxic towards G-CSF-dependent OCI/AML1 cells, but not factor independent OCI/AML3 cells; colony formation by G-CSF-responsive leukemic blasts from a patient with acute myeloblastic leukemia (AML) was also inhibited. The G-CSF fusion toxin displayed ADP-ribosyltransferase activity in a cell-free system. Genetic conjugation of G-CSF to an enzymatically inactive DT mutant, CRM197, resulted in a 200-fold reduction in the ability of G-CSF to stimulate normal bone marrow colony formation. These results suggest that fusion of G-CSF to DT sequences interferes with some of the activity but not the specificity of the ligand binding domain of the molecule. Nevertheless, DAB486-G-CSF may be included with the increasing number of other toxin-hormone fusion proteins whose toxicity is directed towards specific receptor-bearing cells, and may represent a novel approach towards the study and treatment of leukemia.

Expression and dynamic modulation of the human granulocyte colony-stimulating factor receptor in immature and differentiated myeloid cells

In this study we have examined the expression and modulation of the human granulocyte colony-stimulating factor (G-CSF) receptor (R) in immature and differentiated myeloid cells using a 125I labelled human G-CSF analogue (TG50). Equilibrium binding data revealed a single affinity class of receptor on all cell types expressing G-CSFR (KD 235-606 pM) with neutrophils expressing 2883 +/- 672 Rs/cell. Rapid internalization of surface receptor-bound ligand at 37 degrees C was detected in both immature cells (U937) and neutrophils with > 70% of specifically bound ligand internalized within 5 min. Concentration-response data showed that the level of occupancy of neutrophil G-CSFRs by ligand at 37 degrees C was approximately 5-fold greater than predicted by equilibrium binding data and correlated closely with concentration-response data for biological activity. Re-expression of G-CSFRs following down-regulation by internalization was not detected. Down-regulation of the neutrophil G-CSFR by several agents including granulocyte-macrophage colony-stimulating factor (GM-CSF), tumour necrosis factor (TNF), lipopolysaccharide (LPS), f-met-leu-phe (fMLP), phorbol ester (TPA) and C5a was observed at 37 degrees C but not at 4 degrees C. In contrast, G-CSFRs on immature myeloid cells were significantly down-regulated by TPA only. Differentiation of myeloid leukaemic cell line HL-60 with DMSO, a frequently used model of granulocytic differentiation, was associated with a significant reduction in G-CSFR expression (11 +/- 5% of control) whereas treatment with retinoic acid led to increased G-CSFR expression (161 +/- 3%).

Characterization of a saporin mitotoxin specifically cytotoxic to cells bearing the granulocyte-macrophage colony-stimulating factor receptor

When granulocyte-macrophage colony-stimulating factor (GM-CSF) is chemically conjugated to the ribosome-inactivating protein saporin, the resulting protein conjugate is highly toxic for cells expressing the GM-CSF receptor. Structural and Western blot analyses of the purified conjugate establish that it contains equimolar amounts of the starting materials and is free of any contamination by the non-conjugated components. The resulting bifunctional reagent is specifically cytotoxic to cells expressing the GM-CSF receptor, but is ineffective to cells that do not express the receptor. The cytotoxic activity is inhibited in a dose-dependent manner by GM-CSF, but not by any one of five other peptide growth factors. This is the first report of a mitotoxin for cells that express the GM-CSF receptor and which promises to be a valuable tool to study the expression of the GM-CSF receptor in normal and pathological states.

Bone marrow cells from A/J mice do not proliferate in interleukin-3 but express normal numbers of interleukin-3 receptors

Haemopoietic cells from A/J mice do not form colonies (proliferate) in response to interleukin-3 (multi-CSF, IL-3). We have examined different populations of cells from A/J mice and shown that, despite their failure to proliferate in response to IL-3, cells from bone marrow, spleen and the peritoneum all bound 125I-labelled IL-3. A wide variety of cell types bound IL-3 as determined by autoradiography, including promyelocytes, myelocytes, metamyelocytes, polymorphs, promonocytes, monocytes, eosinophils and lymphocytes, but not nucleated erythroid cells, and the proportion of each cell type binding label was similar when cells from A/J mice were compared with those of C57B1/6 and Balb/c mice. Bone marrow cells from A/J mice internalized interleukin-3 with normal kinetics and mRNA extracted from these cells contains the same species of IL-3 receptor and IL-3 receptor-like mRNAs as are found in the other strains. Collectively the data suggest that the failure of haemopoietic cells from A/J mice to proliferate in response to IL-3 is related to a selective defect in signalling to proliferation specific genes. This defect is apparently not related to internalization or processing of the IL-3/IL-3-receptor complex, but may be due to failure to activate appropriate accessory molecules in the cell.

Biochemical events accompanying macrophage activation and the inhibition of colony-stimulating factor-1-induced macrophage proliferation by tumor necrosis factor-alpha, interferon-gamma, and lipopolysaccharide

Agents that can arrest cellular proliferation are now providing insights into mechanisms of growth factor action and how this action may be controlled. It is shown here that the macrophage activating agents tumor necrosis factor-alpha (TNF alpha), interferon-gamma (IFN gamma), and lipopolysaccharide (LPS) can maximally inhibit colony stimulating factor-1 (CSF-1)-induced, murine bone marrow-derived macrophage (BMM) DNA synthesis even when added 8-12 h after the growth factor, a period coinciding with the G1/S-phase border of the BMM cell cycle. This inhibition was independent of autocrine PGE2 production or increased cAMP levels. In order to compare the mode of action of these agents, their effects on a number of other BMM responses in the absence or presence of CSF-1 were examined. All three agents stimulated BMM protein synthesis; TNF alpha and LPS, but not IFN gamma, stimulated BMM Na+/H+ exchange and Na+,K(+)-ATPase activities, as well as c-fos mRNA levels. IFN gamma did not inhibit the CSF-1-induced Na+,K(+)-ATPase activity. TNF alpha and LPS inhibited both CSF-1-stimulated urokinase-type plasminogen activator (u-PA) mRNA levels and u-PA activity in BMM, whereas IFN gamma lowered only the u-PA activity. In contrast, LPS and IFN gamma, but not TNF alpha, inhibited CSF-1-induced BMM c-myc mRNA levels, the lack of effect of TNF alpha dissociating the inhibition of DNA synthesis and decreased c-myc mRNA expression for this cytokine. These results indicate that certain biochemical responses are common to both growth factors and inhibitors of BMM DNA synthesis and that TNF alpha, IFN gamma, and LPS, even though they all have a common action in suppressing DNA synthesis, activate multiple signaling pathways in BMM, only some of which overlap or converge.

The biology of granulocyte-macrophage colony-stimulating factor: effects on hematopoietic and nonhematopoietic cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is one of a family of glycoprotein cytokines that have potent effects in stimulating the proliferation, maturation, and function of hematopoietic cells. Deriving its name from its ability to stimulate the formation of macroscopic colonies containing neutrophils, eosinophils, macrophages, or mixtures of these cell types, GM-CSF stimulates the proliferation and maturation of myeloid progenitors, as well as functionally activating mature neutrophils, eosinophils, and macrophages. As most of the effects observed using GM-CSF in vitro have been shown to occur in vivo either in animal models or in human subjects, it is important to consider that GM-CSF may also exert some biological effects on nonhematopoietic cells. In response to immunologic stimuli, immunologic surveillance cells and cells of the microenvironment are capable of producing GM-CSF. In vitro experiments indicate that GM-CSF production is tightly regulated. In that regard, GM-CSF is not present in measurable quantities in normal serum, but little is known about the in vivo process of GM-CSF production and regulation. The biologic capabilities of GM-CSF have triggered its widespread clinical use in situations where hematopoiesis is compromised. GM-CSF can act as a potent growth factor in vivo, increasing the number and enhancing the function of hematopoietic progenitors and mature cells. However, the precise in vivo effect that GM-CSF may have on normal and neoplastic cells of nonhematopoietic origin remains undefined. The full range of GM-CSF bioactivity is mediated following binding to its receptor. The presence of specific receptors for GM-CSF has been demonstrated in all responsive cells of hematopoietic lineage, as well as in nonhematopoietic cells, both responsive and unresponsive. In conclusion, a large body of work from a number of laboratories has defined the biology of GM-CSF. Currently available reagents and technology will provide additional insights into the biology of this molecule, thereby expanding our present definition and allowing us to explore the mechanisms regulating hematopoiesis.

Synergistic suppression: anomalous inhibition of the proliferation of factor-dependent hemopoietic cells by combination of two colony-stimulating factors

Cells of the continuous murine hemopoietic cell line FDC-P1 expressing macrophage-colony-stimulating factor (M-CSF) receptors following retroviral insertion of murine c-fms cDNA proliferated clonally when stimulated by granulocyte/macrophage (GM)-CSF, multipotential CSF, or M-CSF. However, M-CSF combined with either GM-CSF or multi-CSF, even at low CSF concentrations, strongly inhibited colony formation, with loss of clonogenicity in affected cells accompanied by increased macrophage differentiation. Stimulation by these CSF combinations did not induce short-term changes in CSF receptor expression or internalization. FDC-P1 cells expressing another inserted tyrosine kinase receptor, basic fibroblast growth factor receptor, did not exhibit suppression when GM-CSF was combined with fibroblast growth factor. This phenomenon of synergistic suppression may have relevance for the future clinical use of combinations of CSFs, because a potentially similar suppression is also observable with some normal macrophage progenitor cells.

Rate transition and regulatory coupling in endocytosis of interferon-alpha and tumor necrosis factor-alpha in human epithelial tumor cells

The time-dependent concentrations of interferon-alpha and tumor necrosis factor-alpha associated with the membrane and internalized by cells contain information on the kinetics of endocytosis and their cellular processing. This information can be reduced quantitatively by application of the respective compartmental models. In our studies of human epithelial tumor cells interacting with human interferon-alpha and human tumor necrosis factor-alpha, we accounted only for actual endocytosis and elimination of the tracer from cells by a novel method sensitive to changes in the rate of endocytosis, to the delay in tracer elimination, and to the nonlinear regulatory coupling between endocytosis and the internalized ligand. Data reduced by this method resulted in best-fit parameter values statistically superior to values obtained by previous methods (Bajzer et al., 1989). The results indicate a change with time in the rate of endocytosis of tumor necrosis factor-alpha and the inhibition of endocytosis by the endocytosed ligand-receptor complex. We conclude that sorting and processing of interferon-alpha and tumor necrosis factor-alpha are restricted by the type of both the receptor and the cell.

Affinity conversion of receptors for colony stimulating factors: properties of solubilized receptors

Murine receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF) and Multi-CSF (interleukin-3) can exist in both high- and low-affinity forms and demonstrate trans-modulation by several different ligands. In contrast the recently cloned human GM-CSF receptor and murine interleukin-3 (IL-3) receptor display only low-affinity binding. To begin to understand the molecular basis of the formation of high- and low-affinity receptors and their trans-modulation we have developed methods for the solubilization and assay of GM-CSF and interleukin-3 receptors so that their binding characteristics can be studied in cell-free solution. Both receptors displayed a single class of high-affinity binding on intact FDC-P1 cells and IL-3 receptors had unaltered binding characteristics in cells, membranes and in detergent solution. However, GM-CSF receptors were converted to a single class of low-affinity binding in detergent solution while both high- and low-affinity forms were evident in membranes. The basis of affinity conversion of GM-CSF receptors was exclusively a change in the kinetic dissociation rate of ligand. Cross-linking experiments suggested that high-affinity receptors for GM-CSF and IL-3 might consist of two different protein species and, if this is so, the data suggest that this association is more stable for IL-3 than for GM-CSF receptors.

Binding, internalization and degradation of radio-iodinated interleukin 3 and granulocyte-macrophage colony stimulating factor by various hemopoietic cells

The proliferation and differentiation of hemopoietic committed progenitor cells depend on colony stimulating factors (CSF). However, isolated mouse granulocyte-macrophage progenitor cells can still undergo limited proliferation in serum-free cultures after CSF deprivation. To test whether this is due to an accumulated pool of internalized factor, we examined the binding, internalization and degradation of radiolabelled interleukin 3 (IL-3) and granulocyte-macrophage colony stimulating factor (GM-CSF) in various hemopoietic cells. We found 20,000 high affinity IL-3 receptors on cells of two IL-3-dependent hemopoietic cell lines, FDC-P1 and FDC-P2 (Kd = 85 and 129 pM). FDC-P1 cells, which also respond to GM-CSF, possess 600 high-affinity GM-CSF receptors (Kd = 64 pM). Cells of both lines internalize IL-3, but only FDC-P1 cells release degraded IL-3 at a rapid rate. Both cell lines have similar dose-response curves for IL-3 and survival kinetics after factor removal. All other cells tested behave like FDC-P1, suggesting that the metabolism of IL-3 by FDC-P2 is exceptional. Our study indicates that transient proliferation of committed progenitor cells in the absence of added factors is apparently not due to a stable pool of internalized CSF but merely represents an intrinsic capability of these cells.

Low-affinity placenta-derived receptors for human granulocyte-macrophage colony-stimulating factor can deliver a proliferative signal to murine hemopoietic cells

Retrovirally mediated introduction of a cDNA encoding a placenta-derived low-affinity receptor for human granulocyte-macrophage colony-stimulating factor (GM-CSF) into murine FDC-P1 hemopoietic cells allowed these cells to proliferate when stimulated by human GM-CSF. The expressed human receptors on cloned lines were of low affinity (Kd = 4-6 nM), were internalized, and did not interact with endogenous GM-CSF receptors. Concentrations of human GM-CSF of 6.5-13 nM were required to stimulate 50% maximal colony formation versus a concentration of murine GM-CSF of 6 pM; this difference is comparable with the difference in relative affinities of the human and murine receptors for their respective ligands. If maintained in murine GM-CSF, cells able to bind or respond to human GM-CSF were rapidly lost due to transcriptional inactivation of the inserted cDNA. The observations indicate that low-affinity receptors for human GM-CSF can deliver a proliferative signal in appropriate cells and that the signaling mechanisms are not species-specific.

The induction and inhibition of differentiation in normal and leukaemic cells

For granulocytic-macrophage progenitor populations and their progeny, five glycoproteins have been identified: GM-CSF, G-CSF, multi-CSF, M-CSF and IL-6 that can regulate their proliferative activity, maturation and functional activities. The same glycoproteins also have a capacity to induce irreversible differentiation commitment in normal bipotential granulocyte-macrophage progenitors and in some myeloid leukaemic cell lines, which suggests that common cellular processes exist in both situations. The leukaemia inhibitory factor (LIF) is a glycoprotein, with intriguing properties, which can either induce differentiation in some myeloid leukaemic cell lines or prevent differentiation in normal totipotential embryonic stem cells. The data from the LIF studies suggest a genetic mechanism controlling self-generation that is relatively simple and may be common to all cells. However, the actual cellular response observed appears to depend on the nature of the responding cell.