Onset of TCR-P Gene Rearrangement and Role of TCR-P Expression During CD3-CD4-CD8- Thymocyte Differentiation'

TCR-/3 gene rearrangement or expression is necessary and sufficient for the progression of early ap thymocyte differentiation from the CD3-CD4-CD8triple negative (TN)3 to the CD4+CD8+ double positive stage. The onset of TCR-P rearrangement is currently thought to occur gradually. Some thymocytes were reported to be rearranged at the earliest (CD44+CD25-) TN stage, whereas other thymocytes did not initiate TCR-P rearrangement until the latest (CD44-CD25-) TN stage. Here, we have isolated subsets of TN thymocytes on the basis of surface expression of CD44 and CD25, with c-kit as an additional marker. We present a revised model of early T cell development in which TCR-P and TCR-y rearrangements occur abruptly, at the CD44'owCD25+c-kit'owTN stage. A high level of c-kit expression defines pro-T cells which have not yet rearranged their TCR genes. Germ-line TCR-P transcripts, and transcripts of recombination activating genes (RAG)-1 and 2, are detected before TCR-P gene rearrangement. Analyses of TN thymocytes of RAG-1 mutant mice, and of various TCR mutant and TCR transgenic RAG-1 mutant mice, indicate the existence of a control point at the CD44-CD25'TN stage at which cells expressing a productively rearranged TCR-P chain are selected for further differentiation. lournal of Immunology, 1994, 152: 4783.