The Role of Recombination Activating Gene (<i>RAG</i>) Reinduction in Thymocyte Development in Vivo

Assembly of T cell receptor (TCR)α/β genes by variable/diversity/joining (V[D]J) rearrangement is an ordered process beginning with recombination activating gene (RAG) expression and TCRβ recombination in CD4−CD8−CD25+ thymocytes. In these cells, TCRβ expression leads to clonal expansion, RAG downregulation, and TCRβ allelic exclusion. At the subsequent CD4+CD8+ stage, RAG expression is reinduced and V(D)J recombination is initiated at the TCRα locus. This second wave of RAG expression is terminated upon expression of a positively selected α/β TCR. To examine the physiologic role of the second wave of RAG expression, we analyzed mice that cannot reinduce RAG expression in CD4+CD8+ T cells because the transgenic locus that directs RAG1 and RAG2 expression in these mice is missing a distal regulatory element essential for reinduction. In the absence of RAG reinduction we find normal numbers of CD4+CD8+ cells but a 50–70% reduction in the number of mature CD4+CD8− and CD4−CD8+ thymocytes. TCRα rearrangement is restricted to the 5′ end of the Jα cluster and there is little apparent secondary TCRα recombination. Comparison of the TCRα genes expressed in wild-type or mutant mice shows th