The expression and sequences of T cell antigen receptor β-chain genes in the thymus at an early stage after sublethal irradiation

The sequential appearance of the thymocyte subpopulations and TCR gene messages occurred in the thymus of AKR mice (H-2(k), M1s^a) from 7 to 14 days after sublethal irradiation. The thymocytes on day 7 after irradiation were composed of a large number of CD4⁺CD8⁺ blast-like cells and a relatively high proportion of CD4^-CD8^- cells (15 to 25%) but few CD3(high)CD4⁺CD8^-/CD4^-CD8⁺ cells. Approximately 22% of the CD4^-CD8^- cells were CD3(high) and -27% of the CD3(high)CD4^-CD8^- cells (-6% of whole CD4^-CD8^- cells) were F23.1⁺. The thymocytes on day 7 expressed a large amount of γ- and δ-chain gene transcripts but reduced levels of α- and β-chain gene transcripts. The V gene repertoire of 18 functional β-chain cDNA derived from the thymocytes on day 7 was compared with those of 20 functional β-chain cDNA derived from the thymocytes on day 14 which were composed of a large number of CD3(low)CD4⁺CD8⁺ small-sized cells and a small number of CD3(high)CD4⁺CD8^- cells. It is noteworthy that the distribution of Vβ genes expressed in the thymocytes on day 7 was much the same as that in the thymocytes on day 14 but significantly different from that in normal BALB/c thymocytes as previously described. Interestingly, neither V(β8.1) nor V(β6) genes, which are important for recognition of the product of the M1s^a locus, was detected in these two cDNA libraries. These results suggest that clonal selection of TCR V(β) repertoire, irrespective of positive or negative selection, appears to occur at the early stage of T cell differentiation, i.e., on the blast-like CD4⁺CD8⁺ thymocytes.