Abstract
A decrease in responsiveness to granulocyte colony-stimulating factor (G-CSF) has been implicated in the pathophysiology of cyclic hematopoiesis. Using the canine model of cyclic neutropenia, we examined the response of neutrophil precursors to G-CSF in vitro and G- CSF receptor expression in neutrophils from grey collie dogs to determine whether the abnormal response observed to G-CSF in vivo in this disorder is present at the level of the progenitor cell and is caused by defective G-CSF receptor expression. Bone marrow mononuclear cells from grey collie dogs required sevenfold higher G-CSF concentrations than normal dog cells to achieve half-maximal colony growth [56 pmol/L v 8 pmol/L). Receptor binding assays with 125I- labeled G-CSF and Scatchard analyses of the equilibrium binding data were consistent with expression of a single class of high-affinity receptors for G-CSF on neutrophils from both normal dogs and grey collies with similar receptor numbers (56 to 446 sites/cell v 78 to 199 sites/cell) and binding affinities (28 to 206 pmol/L v 84 to 195 pmol/L). Chemical cross-linking studies identified a G-CSF binding protein of approximately 120 kD on neutrophils from grey collies, similar in size to that on normal dog neutrophils. No abnormal G-CSF receptor mRNA transcripts were detected in neutrophils from grey collie dogs by Northern blot analysis. Treatment of both normal and grey collie neutrophils with G-CSF rapidly induced tyrosine phosphorylation of an 80-kD protein that behaved like canine c-rel. These results demonstrate that the abnormal responsiveness to G-CSF in canine cyclic hematopoiesis is present in neutrophil precursors and is not associated with demonstrable alterations in the number, binding affinity, or overall size of the G-CSF receptor in neutrophils, or with defective tyrosine phosphorylation of p80. These data suggest that cyclic hematopoiesis is caused by a defect in the G-CSF signal transduction pathway at a point distal to G-CSF receptor binding that does not involve the early biochemical events leading to p80 tyrosine phosphorylation.
