Physiological upregulation of RAG1 expression compromises the survival of CD4+CD8+ thymocytes in vitro.

Thymocytes upregulate expression of recombination activating genes (RAGs) upon developmental transition from the CD4-CD8- double-negative to the CD4+CD8+ double-positive (DP) stage. Although enhanced RAG expression is necessary to generate a complete Tcra repertoire, here we demonstrate that elevated RAG expression simultaneously compromises DP thymocyte survival in vitro. Using a series of mouse models in which the expression and activity of RAG proteins varied, we attribute this apparent toxicity to elevated expression of RAG1 protein, independent of its association with RAG2 and its canonical role as a nuclease for V(D)J recombination. We propose that this toxicity accounts for the previously documented expansion of the DP compartment in mice with reduced RAG expression in vivo. These results indicate that DP thymocytes endure RAG1-compromised survival as a necessary trade-off for the formation of a complete Tcra repertoire and suggest novel aspects of RAG1 biology unrelated to V(D)J recombination.