The soluble GM-CSF receptor is produced by human leukemic cell lines.

Equal volumes of supernatant conditioned by the cell lines as shown were applied to a GM-CSF ligand affinity column and eluted fractions were pooled, dialyzed against distilled H₂0, lyophilized, resuspended in 500 μL PBS and studied as described. Shown are representative results of n = 3 separate preparations. The starting volume of supernatant for the illustrated results was 300 mL. Panel A: 25 μL sample from each cell line was used in ¹²⁵I-GM-CSF solution phase binding assays in the absence or presence of a large excess of unlabeled GM-CSF. Specifically bound radioactivity (spec bound, cpm) was calculated by subtracting the precipitated radioactivity in the absence of unlabeled GM-CSF from that in the presence of unlabeled GM-CSF. Bars represent the mean and SEM of duplicate experiments. Panel B: 30 μL sample from each cell line was subjected to 10.5% SDS PAGE under reducing conditions and transferred to PVDF membrane. Immunoblotting was performed with anti-GMRα antibody 8G6. Positive control (+ve) was ligand affinity purified recombinant solGMRα. The position of the molecular weight markers is shown in kilodaltons.