Purification and characterization of a thymocyte growth factor. 2. Biological activity of the thymocyte growth factor.

A thymocyte growth factor, THGF, purified from the supernatant of a thymic T-lymphocyte-precursor cell line (TC.SC-1/2.0), stimulated the growth of splenocytes and thymocytes that had not been activated by a mitogen. Addition of suboptimal doses of mitogen or phorbol myristate acetate did not enhance the response. The growth factor could stimulate the growth of thymocytes synergistically with interleukin 2 but its action was not mediated by interleukin 2 or by interleukin 4. L3T4-Lyt2- thymocytes, Ig- splenocytes, Ig+ splenocytes, and alpha 1-thymosin-sensitive splenocytes were targets for THGF, which also stimulated the formation of haemopoietic spleen colonies in irradiated mice.