Production of soluble granulocyte colony-stimulating factor receptors from myelomonocytic cells

It has been speculated that a soluble form of G-CSFR might be physiologically present in humans, since G-CSFR mRNA that lacks a transmembrane domain has been identified from a human myelomonocytic cell line. Here, we demonstrate human soluble G-CSFR (sG-CSFR) of two different molecular sizes (80 and 85 kDa) on an immunoblot analysis using Abs generated against the amino-terminal, extracellular domain of the full-length G-CSFR. Both isoforms of sG-CSFR were able to bind recombinant human G-CSF (rhG-CSF). RT-PCR analysis with primers targeted outside of the transmembrane region revealed that membrane-anchored G-CSFR is expressed at all maturation stages of purified myeloid cells, including CD34⁺CD13⁺ cells (blasts), CD11b^- CD15⁺ cells (promyelocytes or myelocytes), CD11b⁺CD15⁺ cells (metamyelocytes and mature neutrophils), and CD14⁺ cells (monocytes). On the other hand, sG-CSFR mRNA was detectable in CD11b^-CD15⁺, CD11b⁺CD15⁺, and CD14⁺ cells, but not in the CD34⁺CD13⁺ blast population. The serum concentration of both isoforms of sG-CSFR appeared to be correlated with the numbers of neutrophils/monocytes before and after rhG-CSF treatment in normal individuals. Thus, two isoforms of SG-CSFR are physiologically secreted from relatively mature myeloid cells and might play an important role in myelopoiesis through their binding to serum G-CSF.