DYNAMIC MODULATION OF THE CELL-SURFACE EXPRESSION OF THE GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR-RECEPTOR

The GM-CSF receptor (GM-CSFR) is composed of alpha and beta subunits. Surface expression of the alpha chain alone leads to low affinity GM-CSF binding and of both subunits to high affinity binding; the beta chain is required for transducing a proliferative signal. Studies of GM-CSFR expression have concentrated largely on static events occurring under conditions of binding equilibrium. We have examined the dynamic regulation of high and low affinity GM-CSFR expression in neutrophils (1100 +/- 200 R/cell, K(D) 50 +/0 15 pm) and a GM-CSF dependent human leukaemic cell line, TF-1 (2000 +/- 450 R/cell K(D) 15 +/- 5 pm) and 8600 +/- 1150 R/cell K(D) 1-8 +/- 0.3 nm). The addition of GM-CSF to TF-1 cells (350 pm, 4 h at 37-degrees-C) caused a reduction in subsequent binding of I-125-GM-CSF at low ligand concentration (100 pm) (following a low pH wash to remove surface bound ligand) to 16 +/- 4% and a reduction in binding at high ligand concentration (2 nm I-125-GM-CSF) to 36 +/- 9% of control. Scatchard analysis showed complete down-regulation of high affinity GM-CSFR and a significant reduction in low affinity GM-CSFR. In neutrophils, concentration-response curves of ligand induced receptor down-regulation at 37-degrees-C showed that observed down-modulation was more than 10-fold greater than predicted by static equilibrium binding data and correlated closely with GM-CSF priming of the neutrophil respiratory burst. The addition of IL-3 to TF-1 cells at 37-degrees-C reduced 100 pm I-125-GM-CSF binding to 18 +/- 4% and 2 nm I-125-GM-CSF binding to 46 +/- 5% of control. TF-1 cells, but not neutrophils, were able to re-express GM-CSFR following removal of GM-CSF from medium. TF-1 proliferation assays showed that pulsed GM-CSF (0.35-3.5 nm) for up to 4 h did not cause a significant increase in H-3-thymidine incorporation which required the continued presence of GM-CSF (control 2875 +/- 208 cpm, pulsed GM-CSF 5 ng/ml 4972 +/- 1344, continuous GM-CSF 5 ng/ml 17249 +/- 2982). Therefore, proliferation of TF-1 cells required the continued presence of GM-CSF at a time when there was no detectable surface high affinity GM-CSFR. This shows that signal transduction can take place via the GM-CSFR at a time when there is no detectable high affinity GM-CSF binding and introduces a further layer of complexity in the analysis of GM-CSF receptor function.