Thymus-conditioned medium influences in vitro colony formation by hematopoietic progenitor cells

Medium conditioned by mouse thymic cells cultured for 7 days in the presence of concanavalin A (Con A) and 5% human serum enhanced erythroid colony formation by CFU-E but not BFU-E. Medium conditioned by thymic cells for less than 7 days or more than 7 days inhibited erythroid colony formation. Enhancement of erythroid colony formation occurred at both suboptimal and saturating concentrations of erythropoietin. The thymus-conditioned medium also stimulated the growth of granulocyte/macrophage colonies by itself and in the presence of a saturating concentration of WEHI-3 conditioned medium. The conditioned medium contained neither burst-promoting activity nor erythropoietin. The erythroid colony-enhancing activity in the thymus-conditioned medium was heat stable, bound to immobilized Con A, lacked protease activity and migrated with an approximate molecular weight of 68,000 on gel filtration.