Effect of thymosin on glucocorticoid receptor activity and glucocorticoid sensitivity of human thymocytes.

Incubation with thymosin fraction 5, (TMS F5 at 300 micrograms/ml) a partially purified thymic factor, reduced the steroid binding activity of human infant thymocytes from 9.6 +/- 2.1 fmole/ml to 5.0 +/- 2.0 fmole/ml. The glucocorticoid receptor activity in normal infant thymocytes was found to be 2,146 +/- 726 (s.d.) sites per cell with dissociation constant of 1.4 +/- 0.6 X 10(-8)M. TMS F5 also increased the resistance of human thymocytes to the cytolytic effect of dexamethasone (2.5 X 10(-8)M) to 168.6 +/- 30.2% of control (P less than 0.01). In animals, medullary and peripheral blood T cells are more resistant to glucocorticoids than immature thymic T cells. The results show that thymosin can induce changes consistent with differentiation in human thymocytes. These in vitro results are consistent with a physiological role of thymosin in intrathymic T cell maturation in man. Incubation of a human malignant thymus derived T cell line (MOLT 3) with TMS F5 also resulted in a significant reduction of the number of steroid binding sites to 44.2 +/- 15.3% of control (P less than 0.05), but TMS F5 did not significantly reduce the glucocorticoid sensitivity of MOLT 3 cells.