Thymus regeneration by bone marrow cell suspensions differing in the potential to form early and late spleen colonies

We have determined the thymus-repopulating capacity of purified hemopoietic stem cells, bone marrow cells from mice injected four days previously with 5-fluorouracil (5-FUBM), and bone marrow cells cultured in the presence of stem-cell-activating factor (SAF; SAFBM). SAF is identical to interleukin 3 (IL-3). Purified stem cells are more enriched in day-12 CFU-S than in day-8 CFU-S. 5-FUBM consists of CFU-S that give rise to late (day-12) spleen colonies. SAFBM contains predominantly CFU-S that give rise to early spleen colonies (days 6-8). There is also a net increase in the number of spleen colony-forming units (CFU-S) in these cultures. Thymus regeneration after transplantation with either purified stem cells or 5-FUBM was delayed approximately three days as compared with that after transplantation with normal bone marrow cells. This delay can be ascribed to the absence of prothymocytes in these preparations. Thymus regeneration by SAFBM was delayed approximately ten days as compared with that after transplantation with normal bone marrow cells. The most likely explanation of these results is as follows. The prothymocytes in normal bone marrow produce a relatively limited offspring in the thymus soon after transplantation. This is rapidly replaced by the offspring of newly formed prothymocytes, the results of differentiation of the pluripotent stem cells. These stem cells also give rise to late spleen colonies. Stem cells that give rise to early spleen colonies appear to have lost the capacity for differentiation into the T-cell lineage.