Identification and Molecular Cloning of a Soluble Human Granulocyte-Macrophage Colony-Stimulating Factor Receptor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) plays an important role in hematopoiesis and host defense via interaction with specific cell-surface receptors in target tissues. We identified a truncated, soluble form of the low-affinity GM-CSF receptor (GMR) in choriocarcinoma cells. Low-affinity GMR cDNAs encoding both the membrane-bound and soluble receptors were obtained by PCR using primers corresponding to the published sequence. Clones encoding the soluble receptor were identical in sequence to the membrane-bound form but contained a 97-nucleotide internal deletion. The amino acid sequence of this deleted cDNA predicts a protein that lacks the 84 C-terminal amino acids of the membrane-bound receptor, including the transmembrane and cytoplasmic domains, and contains 16 different amino acids at its C terminus. Expression of the soluble GMR cDNA in murine ψ-AM cells as well as GM-CSF-dependent myeloid 32Dc13 cells produced a secreted protein that retained its capacity to bind GM-CSF in solution. RNase protection analysis indicates that this variant cDNA is derived from a naturally occurring mRNA. Soluble receptors have been identified for several other hematopoietin receptors and may be a general feature of this class. The striking similarity between the soluble form of the GMR and other hematopoietin receptors suggests that soluble binding proteins may play an important role in regulating the broad spectrum of biological responses mediated by these cytokines.