Direct stimulation by purified GM-CSF of the proliferation of multipotential and erythroid precursor cells.

GM-CSF. purified from mouse lung-conditioned medium. stimulated granulocyte-macrophage colony formation but was unable to stimulate the formation of any type of pure or mixed erythroid colony in agar cultures of 1 2-day CBA fetal liver cells. However. initial incubation of cells for 2 days with purified GM-CSF. followed by addition of mouse spleenconditioned medium (SCM). allowed the formation of up to 50% of the erythroid or mixed hemopoietic colonies expected in cultures stimulated from the outset by SCM. These GM-CSF-responsive colony-forming cells were not distinguishable from the remainder of cells forming erythroid or mixed colonies by sedimentation velocity or cell cycle status. From 302 clones stimulated to proliferate by purified GM-CSF. 25 formed erythroid or mixed hemopoietic colonies on transfer to cultures containing SCM. Clones initiated by SCM were unable to form erythroid or mixed colonies when transferred to cultures containing purified GM-CSF. Single cells from 12-day CBA fetal peripheral blood also were stimulated to proliferate by purified GM-CSF and subsequently formed pure or mixed erythroid colonies when SCM was added. GM-CSF is thus able to directly stimulate up to 5 cell divisions of at least 50% of detectable multipotential and early erythroid precursor cells.