Control of differentiation of thymocyte precursors in the bone marrow by thymic hormones

Terminal deoxynucleotidyl transferase (TdT) is uniquely associated with the major thymocyte population and a minor bone marrow population which is separable on bovine serum albumin gradients. The expression of TdT in the bone marrow subpopulation is under thymic regulation in that this population is significantly reduced in nude mice or is lost with time after thymectomy. The expression of TdT in athymic mice can be induced both in vivo and in vitro by thymosin fraction 5 or by beta3, a peptide purified from thymosin fraction 5. The optimal concentration of beta3 in the in vitro induction of TdT is approximately 10 ng/ml. These results demonstrate that thymic hormones control the early differentiation of prothymocytes in the bone marrow.