Patterns of Thymocyte Labeling in Vitro with Tritiated Thymidine and Deoxycytidine, with Special Reference to Competitive Incorporation of the Two Deoxyribonucleosides

Using short-term cultures of rabbit thymocytes, the effect of nonradioactive (cold) deoxycytidine on 3H-thymidine incorporation by these cells in vitro was studied, together with the effect of cold thymidine on the uptake of 3-deoxycytidine, chiefly by autoradiographic techniques.Without addition of cold deoxycytidine to the culture medium, a fairly high proportion of the thymocytes was heavily labeled with 3H-thymidine. However, the extent of this reaction was strikingly decreased (to 1/5-1/20 of the control levels)by the addition of cold deoxycytidine at oncentrations ranging from 10 to 80μg/ml. On the other hand, no remarkable reduction in rate of 3H-deoxycytidine incorporation by the thymocytes was caused by the addition of cold thymidine to the medium in sufficient amounts (10-80μg/ml). Among the thymocytes, a minor population was identified which was characterized by intense labeling with 3H-thymidine in spite of the addition of cold deoxycytidine to the medium in sufficient amounts. Such thymocytes are considered to correspond to those which have recently been shown to be cortisone-resistant and immunologically active cells by immunological studies