Single-molecule imaging of transcription dynamics, RNA localization and fate in human T cells.

T cells are critical effector cells counteracting infections and malignancies. To achieve this, they produce pro-inflammatory cytokines, including IFN-γ and TNF. Cytokine production is a tightly regulated process, but the relative contribution of transcriptional and post-transcriptional regulation to mRNA expression remains unknown. We optimized single-molecule FISH for primary human T cells (T-cell smFISH) to simultaneously quantify nascent RNA, levels of mature mRNA, and its localization with single-cell resolution. T-cell smFISH uncovered heterogeneous cytokine mRNA levels, with high cytokine producers displaying biallelic IFNG/TNF RNA transcription activity. Throughout activation, nuclear cytokine mRNAs accumulated, whereas cytoplasmic cytokine mRNA was degraded through translation-dependent decay. Lastly, T-cell smFISH uncovered cytokine-specific regulation by the RNA-binding protein HuR. Thus, T-cell smFISH provides novel insights in the intricate (post)-transcriptional processes in T cells.