2 WAVES OF RECOMBINASE GENE-EXPRESSION IN DEVELOPING THYMOCYTES

During T cell development in the thymus, T cell receptor (TCR) alpha, beta, gamma, and delta genes are rearranged and expressed. TCR rearran gement strictly depends upon the coordinate activity of two recombinas e activating genes, Rag-1 and Rag-2. In this study we have followed th e expression of these genes at different stages of intrathymic develop ment. The results indicate that there are two periods of high Rag-1 an d Rag-2 mRNA expression. The first wave peaks early at the CD25+ CD4- CD8- CD3- stage of development and coincides with the initial appearan ce of transcripts derived from fully rearranged TCR beta, gamma, and d elta genes, whereas the second wave occurs later at the CD4+CD8+ stage coincident with full-length TCR alpha mRNA expression. Active downreg ulation of Rag-1 and Rag-2 mRNA expression appears to occur in vivo be tween the two peaks of recombinase activity. This phenomenon can be mi micked in vitro in response to artificial stimuli such as phorbol myri state acetate and calcium ionophore. Collectively our data suggest tha t recombinase expression is actively regulated during early thymus dev elopment independently of cell surface expression of a mature heterodi meric TCR protein complex.