T helper cells exhibit a dynamic and reversible 3'UTR landscape.

3 untranslated regions (3UTRs) are critical elements of messenger RNAs, as they contain binding sites for RNA-binding proteins (RBP) and microRNAs that affect various aspects of the RNA life cycle including transcript stability and cellular localisation. In response to T cell receptor activation, T cells undergo massive expansion during the effector phase of the immune response and dynamically modify their 3UTRs. Whether this serves to directly regulate the abundance of specific mRNAs or is a secondary effect of proliferation remains unclear. To study 3UTR dynamics in T helper cells we investigated division-dependent alternative polyadenylation (APA). We generated 3 end UTR sequencing data from naive, activated, memory and regulatory CD4+ T cells. 3UTR length changes were estimated using a non-negative matrix factorization approach and were compared with those inferred from long-read PacBio sequencing. We found that APA events were transient and reverted after effector phase expansion. Using an orthogonal bulk RNAseq dataset, we did not find evidence of APA association with differential gene expression or transcript usage, indicating that APA has only a marginal effect on transcript abundance. 3UTR sequence analysis revealed conserved binding sites for T cell-relevant microRNAs and RBPs in the alternative 3UTRs. These results indicate that polyA site usage could play an important role in the control of cell fate decisions and homeostasis.