RNA Binding Protein HuR is critical for the regulation of IL-2 Expression in CD4+ T Cells

Abstract

Abstract The RBP HuR regulates T helper cell differentiation and gene expression by binding to AU-rich elements in the 3′ UTR of its target mRNA and modulating localization and stability. We hypothesized that HuR plays an important role in the regulation of the IL-2 signaling during T cell differentiation and activation. To test this hypothesis, we used the conditional dLck-Cre ROSA HuRfl/fl knock-out mouse to delete HuR late in T cell development but prior to T cell activation. The OVA challenge model of airway inflammation was used to ascertain the effects of HuR KO in vivo. While only 50% of CD4+ T cells in HuR KO mice lack HuR, in vivo and in vitro Th2 responses were essentially nonexistent and there were only background levels of lung inflammation using an OVA challenge model of airway inflammation. The few lung infiltrating CD4+ T cells present had reductions in IL-2 and CD25 expression. HuR KO T cells also had profound antigen-specific proliferation defects in vitro compared to OVA immunized controls and non-specific CD3/CD28 stimulation only partially restored proliferation defects. Furthermore, there is a decrease in IL-2Rα (CD25) which results in decreased levels of phosphorylated (active) STAT5 protein and Blimp1 mRNA and protein levels. To further assess HuR role in regulating IL-2, we developed the pSTAT5+ dLck-Cre HuRfl/fl mouse model, containing a transgene producing constitutively active STAT5. In these mice, there is significant pSTAT5 protein in activated CD4+ T cells, which should circumvent IL-2 downstream signaling, and correct the observed phenotypes. pSTAT5+ HuR KO T cells also had alterations in their expression of multiple CD3 subunits. We conclude that HuR is essential for regulating IL-2 homeostasis and T cell differentiation.