Granulocyte‐macrophage colony stimulating factor receptor α and β chain complexes can form both high and intermediate affinity functional receptors

Scatchard analysis of primary human haemopoietic cells using iodinated GM‐CSF suggests that there are low, intermediate and high affinity classes of the GM‐CSF receptor. To investigate the molecular basis of this, we generated a clone of transfected NIH3T3 cells that constitutively expressed the human granulocyte‐macrophage colony stimulating factor receptor (GM‐CSF R) β chain and inducibly expressed the human GM‐CSF R α chain. In the cells fully induced to express the α chain the overall level of expression of the α and β chains at the cell surface was comparable with that found in primary haemopoietic cells and cell lines. When cells were partially induced to express the α chain, the α:β ratio determined by antibody binding was approximately 1:1 and Scatchard analysis revealed a single class of intermediate affinity receptors (Kd = 614 ± 88 pm). In cells with fully induced α chain expression, the α:β ratio was approximately 3:1 and there was a switch to a dual high and low affinity receptor with Kds of 67 ± 32 pm and 1.7 ± 0.56 nm respectively. The change from intermediate to high affinity was not associated with changes in αβ stoichiometry as detected by cross‐linking with radiolabelled GM‐CSF and gel electrophoresis. Both the high and intermediate affinity receptors were able to activate the STAT 5 and the MAP kinase pathways, although there was a difference in the ligand dose–response curves which was compatible with the different affinities of the receptors. It is proposed that the switch from an intermediate to high affinity receptor was due to the availability of free α chains presenting ligand to the αβ chain complexes at the surface of the cell membrane.