"Mature" thymocytes are not glucocorticoid-resistant in vitro.

Dexamethasone, administered in vitro either continuously or for a 30-min pulse period followed by a chase of 18 to 24 hr, is shown to decrease protein synthetic rates as well as cellular viabilities in a dose-dependent manner in murine thymocytes. Differential effects on cortical and medullary thymocyte subpopulations should be detectable by alterations in the rates of synthesis of specific molecules which are stable in vitro, relative to total protein synthesis, in the absence of steroids. However, terminal deoxynucleotidyl transferase (TdT), a marker for cortical cells, and Qa-1 and H-2, preferentially expressed in medullary cells, continue to be produced as constant fractions of total synthesis even after treatment with up to 10(-6) M dexamethasone. Furthermore, thymocytes obtained from normal and in vivo cortisone-treated mice show little, if any, difference in their intrinsic sensitivities to dexamethasone in vitro. The results reported here suggest that a corticoresistant thymocyte per se does not exist in vitro and that the thymus may produce factor(S) in vivo that protect the medullary subpopulation from in vivo glucocorticoid-induced lysis.