ONSET OF TCR-BETA GENE REARRANGEMENT AND ROLE OF TCR-BETA EXPRESSION DURING CD3(-)CD4(-)CD8(-) THYMOCYTE DIFFERENTIATION

TCR-beta gene rearrangement or expression is necessary and sufficient for the progression of early alpha beta thymocyte differentiation from the CD3(-)CD4(-)CD8(-) triple negative (TN)(3) to the CD4(+)CD8(+) do uble positive stage. The onset of TCR-beta rearrangement is currently thought to occur gradually. Some thymocytes were reported to be rearra nged at the earliest (CD44(+)CD25(-)) TN stage, whereas other thymocyt es did not initiate TCR-beta rearrangement until the latest (CD44(-)CD 25(-)) TN stage. Here, we have isolated subsets of TN thymocytes on th e basis of surface expression of CD44 and CD25, with c-kit as an addit ional marker. We present a revised model of early T cell development i n which TCR-beta and TCR-gamma rearrangements occur abruptly, at the C D44(low)CD25(+)c-kit(low)TN stage. A high level of c-kit expression de fines pro-T cells wh ich have not yet rearranged their TCR genes. Germ -line TCR-beta transcripts, and transcripts of recombination activatin g genes (RAG)-1 and 2, are detected before TCR-beta gene rearrangement . Analyses of TN thymocytes of RAG-1 mutant mice, and of various TCR m utant and TCR transgenic RAG-1 mutant mice, indicate the existence of a control point at the CD44(-)CD25(+)TN stage at which cells expressin g a productively rearranged TCR-beta chain are selected for further di fferentiation.