Sequental appearance of thymocyte subpopulations and T cell antigen receptor gene messages in the mouse thymus after sublethal irradiation

The sequential differentiation patterns of thymocyte were observed with cell surface phenotypes and the expression of T cell antigen receptor in 800 rad irradiated adult mice. Thymus was severely reduced in size and cell number by day 5 after whole body irradiation and rapidly recovered from day 7 to 14. Surface marker analysis on day 5 after irradiation showed thymocytes with Thy-1(low) L3T4⁺/Lyt-2^- dominantly existed and suggested that these cells were radioresistant-survived cells. On the other hand, thymocytes on day 7 were composed of a large number of Thy-1(high) L3T4⁺/Lyt-2⁺ blast-like cells and a relatively high proportion of Thy-1(high) L3T4^-/Lyt-2^- cells which expressed a large amount of γ-chain gene messages but scarcely any α- and β-chain gene messages similar to the fetal thymocytes. On day 14, thymocytes were composed mostly of Thy-1(high) H-2(low) L3T4⁺/Lyt-2⁺ subpopulation which expressed a remarkably low level of γ-chain gene messages, and high levels of α- and β-chain transcripts analogous to those of normal adult thymus. Taken together, intrathymic radioresistent stem cells for T thymocytes seem to proliferate and differentiate after irradiation with the same pattern as was seen in a fetal thymus development.