Dynamic 3'UTR changes during naïve CD4+ T cell activation and proliferation are reverted in the effector/memory state [Iso-Seq]

3’ untranslated regions (3’UTRs) are critical elements of most messenger RNAs: they contain binding sites for RNA-binding proteins (RBP) and microRNAs that affect various aspects of the RNA life cycle including transcript stability. In response to T cell receptor activation, T cells undergo massive expansion during the effector phase of the immune response and dynamically modify their 3’UTRs. Whether this serves to directly regulate the abundance of specific mRNAs or is a secondary effect of proliferation remains unclear. To study 3’UTR dynamics in T helper cells we investigated division-dependent alternative polyadenylation (APA). We generated 3’ end UTR sequencing in naïve, 48h activated (anti-CD3/CD28 cultured), memory and regulatory CD4+ Tcells. 48h activated Tcells were CTV-separated into three populations: cells not yet divided, divided once and twice. 3’UTR length changes were estimated using non-negative matrix factorization approach and partially compared with long-read PacBio sequencing. We found that, in T helper cells, APA events were transient and reverted after effector phase expansion. Using an orthogonal bulk RNAseq dataset, we found that APA genes were not significantly associated with either post-transcriptional or transcriptional regulation, differential gene expression or transcript usage, demonstrating a marginal effect of APA on transcript abundance. 3’UTR sequence analysis revealed conserved binding sites for important microRNAs and RBPs co-evolving with APA events. These results highlight the important role of polyA site usage in the control of cell fate decisions and homeostasis. Experimental design includes 3 samples: naive, 48h activated no division and 2 divisions