The soluble GM-CSF receptor is found in plasma conditioned by human hematopoietic progenitor cells.

Plasma conditioned by PBSC products was subjected to Con-A sepharose and ligand affinity column chromatography. Pooled fractions from the ligand affinity column were dialyzed against distilled H₂O lyophilized, and then resuspended in 500 μL PBS for analysis. Samples 1, 5, 6 = donors for autologous transplant. Samples 2, 3, 4 = donors for allogeneic transplant. Panel A: 25 μL sample from each donor 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 donor 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. Negative control (-ve) was sham ligand affinity column eluate. The position of the molecular weight markers is shown in kilodaltons.