STAT3 is an important transcription factor involved in mediating intracellular signals initiated at the cell membrane by cytokines and growth factors. There is ample evidence that STAT3 acts as a positive regulator of cell growth, but most of this information derives from studies done with isolated cells in vitro. Because functional deletion of STAT3 in mice is lethal, it was difficult to evaluate a role for STAT3 in mediating hematopoietic effects in vivo in mice after birth. To address this problem, a unique strain of mice was developed with tissue specific disruption of STAT3 in bone marrow and hematopoietic cells (

). The availability of this conditional STAT3 −/− mouse model demonstrated a critical role for STAT3 in innate immunity. We have now utilized this conditional STAT3 −/− mouse model to evaluate a role for STAT3 in hematopoiesis after birth, with the hypothesis that STAT3 would be one critical factor involved in the proliferation of myeloid progenitor cells (MPC: CFU-GM, BFU-E, CFU-GEMM) in bone marrow and spleen. STAT3 −/− and their littermate control mice were evaluated at 4 weeks of age. STAT3 −/− mice manifested 40–44% decreases in absolute numbers of nucleated cells in the marrow (femur) and spleen. This was associated with decreased absolute numbers of CFU-GM (70%), BFU-E (70%) and CFU-GEMM (50%) per femur and CFU-GM (50%), BFU-E (30%), and CFU-GEMM (50%) per spleen for these MPC which are responsive in vitro to stimulation of colony formation by the combination of EPO, SCF, TPO and growth factors in PWMSCM. Moreover, MPC from STAT3 −/− mice were in a slow or non cycling state (0–4% MPC in S-phase) in marrow and spleen compared to 50–60% marrow and 32–48% spleen MPC from +/+ mice being in active cell cycle. There were also large decreases per femur in STAT3 −/− mice in terms of GM-CSF-, IL-3-, M-CSF-, GM-CSF plus SCF-, GM-CSF plus Flt3 ligand (FL)-, IL-3 plus SCF-, IL-3 plus FL-, M-CSF plus SCF-, and M-CSF plus FL- responsive CFU-GM. These decreases may in part reflect the finding that CFU-GM from STAT3 −/− mice did not respond to the synergistic proliferation effects of GM-CSF, IL-3, or M-CSF, each in combination with either SCF or FL. At best these cytokine combinations resulted in additive proliferative effects on MPC from marrow of STAT3 −/− mice in contrast to CFU-GM from +/+ mouse marrow where the effects were clearly synergistic. In terms of survival of MPC, there were no apparent differences between the survival of MPC from STAT3 −/− and +/+ mice after withdrawal of growth factors in vitro and their delayed addition to the cell cultures. MPC from STAT3 −/− and +/+ marrow responded similarly to the survival enhancing effects in vitro of SDF-1/CXCL12. Our results demonstrate that after birth STAT3 acts as a positive mediator of the proliferation of MPC in vivo, and STAT3 is a critical mediator of the synergistic proliferation effects of cytokines on MPC.

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