Kinetics of responses of IL3 dependent lines to growth factors: implications for the assay and mechanism of cell promotion.

We have investigated the promotion of two IL3 dependent cell lines by IL3 and a rat factor derived from stimulated spleen cells. Both lines respond to purified IL3, but do not respond to IL2, GM-CSF contained in lung conditioned medium nor to Human IL2. Both lines respond to a factor in rat lymphocyte conditioned media as determined by a standard assay used to detect IL3. However, 123 cell line cells continue to proliferate whereas AC2 cells decline, due to different sensitivities of the lines to rat factor. 123 cells can be re-cloned in the rat factor and exhibit enhanced responses to IL3. The changing kinetics of cell promotion during the first 48 hours question the validity of the standard assay which is widely used, and the specificity of the mechanism by which the cells are promoted.