Identification of the soluble granulocyte-macrophage colony stimulating factor receptor protein in vivo

On the basis of the finding of alternatively spliced mRNAs, the alpha-subun it of the receptor for QM-CSF is thought to exist in both a membrane spanni ng (tmGMR alpha) and a soluble form (solGMR alpha). However, only limited d ata has been available to support that the solGMR alpha protein product exi sts,in vivo. We hypothesized that hematopoietic cells bearing tmGMR alpha w ould have the potential to also produce solGMR alpha. To test this hypothes is we examined:media conditioned by candidate cells using functional, bioch emical, and immunologic means. Three human leukemic cell lines that express tmGMR alpha (HL60, U937, THP1) were shown to secrete GM-CSF binding activi ty and a solGMR alpha-specific band by Western blot, whereas a tmGMR alpha- negative cell line (K562) did not. By the same analyses, leukapheresis prod ucts collected for autologous and allogeneic stem cell transplants and medi a conditioned by freshly isolated human neutrophils also contained solGMR a lpha. The solGMR alpha protein in vivo displayed the same dissociation cons tant (Kd = 2-5 nmol) as that of recombinant solGMR alpha. A human solGMR al pha ELISA was developed that confirmed the presence of solGMR alpha in supe rnatant conditioned by the tmGMR alpha-positive leukemic cell lines, hemato poietic progenitor cells, and neutrophils, Furthermore, the ELISA demonstra ted a steady state level of solGMR alpha in normal human plasma (36 +/- 17 pmol) and provided data suggesting that plasma solGMR alpha levels can be e levated in acute myeloid leukemias. (Blood.2000;95:461-469) (C) 2000 by The American Society of Hematology.