In vitro and in vivo induction of terminal deoxynucleotidyl transferase activity in bone marrow cells by thymic humoral factors derived from a tumor cell of thymic epithelial origin.

Thymic epithelial tumor cell supernatant fraction 5 isolated from the cell culture medium of a tumor cell of thymic epithelial origin was tested for its ability to induce TdT expression. TdT activity could be induced in fractionated and unfractionated bone marrow cells from athymic nude rnu/rnu rats by supernatant fraction 5. Incubation with supernatant fraction of 5 rnu/rnu bone marrow cells isolated from the interface between 23% and 26% BSA restored the level of TdT activity to levels normally found in the thymus-bearing rnu/+ littermates. A does-related response was observed in vitro and in vivo in the expression of TdT. TdT induction in vitro was maximal with 5 micrograms/ml supernatant fraction 5 and in vivo with 200 micrograms supernatant fraction 5/rat/day. The effect was specific in that neither saline nor culture medium fraction 5 treated cells were induced to express TdT. The data presented in this study demonstrate that thymic epithelial tumor cell supernatant fraction 5 exerts an influence on the early maturation and differentiation of bone marrow stem cells.