Sequential 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 day 14. Surface marker analysis on day 5 after irradiation showed thymocytes with Thy-1low 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-1high L3T4+/Lyt-2+ blast-like cells and a relatively high proportion of Thy-1high L3T4-/Lyt-2- cells which expressed a large amount of gamma-chain gene messages but scarcely any alpha- and beta-chain gene messages similar to the fetal thymocytes. On day 14, thymocytes were composed mostly of Thy-1high H-2low L3T4+/Lyt-2+ subpopulation which expressed a remarkably low level of gamma-chain gene messages, and high levels of alpha- and beta-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.