Analysis of high molecular weight RNA-induced silencing complex (HMW-RISC) in CD8+ T cells identifies miR-7 as a modulator of T cell activation

Increasing evidence suggests mammalian Argonaute (Ago) proteins partition into distinct complexes within cells, but there is still little functional understanding of the miRNAs differentially associated with these complexes. In naive T cells, Ago2 is found almost exclusively in low molecular weight (LMW) complexes which are associated with miRNAs but not their target mRNAs. Upon T cell activation a proportion of these Ago2 complexes move into a newly formed HMW RISC which is characterized by the presence of the GW182 protein that mediates translational repression. To identify the miRNAs expected to be potent in suppressing targets, we followed HMW RISC formation upon activation of CD8+ T cells. We show that while most miRNAs distribute between HMW and LMW RISC in activated T cells, several miRNAs were dominant in one complex over the other. Among these, miR-7 is enriched in HMW RISC and inhibition of miR-7 upon T cell activation leads to increased production of IL-2 and expression of IL-2-regulated proteins including the α-subunit of the IL-2 receptor, CD25; transferrin receptor, CD71; and amino acid transporter, CD98, which are direct miR-7 targets. Our data support a model where recruitment of miR-7 to HMW RISC restrains IL-2 signalling and the metabolic processes regulated by IL-2 and thus modulates T cell activation.