Determining the biological significance of the preformed GM-CSF receptor

The human GM-CSF receptor is comprised of a ligand specific α chain and a βchain that is shared with the receptors for IL-3 and IL-5. The association of the receptor suhunits for IL-3 and IL-5 is dependent on interaction with cognate ligand. We have shown however that the GM-CSF receptor suhunits exist in two forms; one form in which association of α chain and β chain is GM-CSF dependent and the other, a preformed complex, independent of GM-CSF binding. The exact significance of this ohservation is not clear but it may be of fundamental importance for understanding haemopoiesis, particularly as the preformed GM-CSF receptor appears to he recruited into IL-3 and IL-5 induced receptor complexes. We are using a dominant negative approach to study the role of the preformed GM-CSF receptor in the proliferation and differentiation of committed myeloid progenitors. We have made a retroviral construct that expresses a cytoplasmically truncated GM-CSF receptor a chain. We have shown in transfeclants that this truncated a chain binds GM-CSF with normal binding characteristics and complexes with β chain to form high affinity binding sites. However, this truncated ex chain does not mediate β chain phosphorylation and moreover, blocks phosphorylation of the normal receptor ie. acts as a dominant negative. We are transducing this retroviral construct into CD34+ CD38 human progenitors and examining the effect on colony formation and proliferation. We hope this approach will enable us to determine the role of GM-CSF preformed receptor in lineage commitment and clonal expansion.