PU.1 REGULATES BOTH CYTOKINE-DEPENDENT PROLIFERATION AND DIFFERENTIATION OF GRANULOCYTE MACROPHAGE PROGENITORS/

PU.1 is a unique regulatory protein required for the generation of bot h the innate and the adaptive immune system. It functions exclusively in a cell-intrinsic manner to control the development of granulocytes, macrophages, and B and T lymphocytes. We demonstrate that mutation of the PU.1 gene causes a severe reduction in myeloid (granulocyte/macro phage) progenitors, PU.1 -/- myeloid progenitors can proliferate in vi tro in response to the multilineage cytokines interleukin-3 (IL-3), IL -6 and stem cell factor but are unresponsive to the myeloid-specific c ytokines granulocyte-macrophage colony-stimulating factor (GM-CSF), G- CSF and M-CSF, The failure of PU.1 -/- progenitors to respond to G-CSF is bypassed by transient signaling with IL-3, In the presence of IL-3 and G-CSF, PU.1 -/- progenitors can differentiate into granulocytic p recursors containing myeloperoxidase-positive granules. Thus PU.1 is n ot essential for specification of granulocytic precursors, but is requ ired for their further differentiation. The failure of PU.1 -/- progen itors to respond to M-CSF is due to lack of c-fms gene transcription. Transduction of c-fms into PU.1 -/- myeloid progenitors bypasses the b lock to M-CSF-dependent proliferation but does not induce detectable m acrophage differentiation. Therefore, PU.1 appears to be essential for specification of monocytic precursors. Importantly, retroviral transd uction of PU.1 into mutant progenitors restores responsiveness to myel oid-specific cytokines and development of mature granulocytes and macr ophages. Thus PU.1 controls myelopoiesis by regulating both proliferat ion and differentiation pathways.