Abstract
We studied the effects of interleukin‐3 (IL‐3) on colony formation by hemopoietic progenitors in methylcellulose cultures of spleen cells from 5‐fluorouracil (FU)‐treated mice. Purified IL‐3 supported the growth of various types of multilineage colonies including blast cell colonies. The types of colonies were similar to those supported by pokeweed‐mitogen spleen cell conditioned medium (PWM‐SCM), except that IL‐3 supported eosinophil and neutrophil expression better. Delayed addition of IL‐3 to cultures 7 days after cell plating decreased the number of colonies to one‐half the number in cultures with IL‐3 added on day 0. It did not alter the proliferative and differentiation characteristics of late emerging multipotential blast cell colonies. These observations suggest that IL‐3 does not trigger hemopoietic progenitors into active cell proliferation but is necessary for their continued proliferation. This permissive role of IL‐3 is consistent with a stochastic model of stem cell proliferation which features random entry into cell cycle. IL‐3 also supported the growth of multilineage colonies from single cells isolated from blast cell colonies by micromanipulation. This result shows that IL‐3 acts directly on multipotential progenitors. Analysis of colonies derived from paired progenitors revealed disparate lineage expression and was in accordance with the stochastic model of stem cell differentiation.