UCSF

Helper T cells play a critical role in orchestrating and maintaining a healthy immune system. During an immune response, antigen-specific CD4+ T cells proliferate and differentiate into effector T cells that act to coordinate the adaptive immune response by selective cytokine production. Thus, naïve T cells transition from a state of quiescence in which they express markers required for trafficking between blood and lymphoid organs, to an effector T cell fate, in which they actively divide, secrete effector molecules, and traffic to sites of infection. Major changes in gene expression characterize this transition, and microRNAs (miRNAs) play a critical role in this process. MiRNA expression also changes dramatically during T cell differentiation and in this dissertation, we strove to characterize these expression changes and investigate the underlying mechanism.

We used quantitative analyses to determine that T cell activation induces global post-transcriptional miRNA downregulation in vitro and in vivo. Argonaute (Ago) proteins, the core effector proteins of the miRNA-induced silencing complex (miRISC) are post-transcriptionally downregulated during T cell activation. We found that Ago2 is inducibly ubiquitinated in activated T cells, and its downregulation is inhibited by the proteasome inhibitor MG132. We hypothesize that activation-induced miRNA downregulation likely occurs at the level of miRISC turnover. We also identified an additional layer of activation-induced miRNA-specific transcriptional regulation. Thus, transcriptional and post-transcriptional mechanisms cooperate to rapidly reprogram the miRNA repertoire in differentiating T cells. Altering Ago2 expression in T cells revealed that Ago proteins are limiting factors that determine miRNA abundance. Naïve T cells with reduced Ago2 and miRNA expression differentiate more readily into cytokine-producing helper T cells, suggesting that activation-induced miRNA downregulation promotes acquisition of helper T cell effector functions by relaxing the repression of genes that direct T cell differentiation.