RNA Binding Protein Hur Regulates CD4+ T Cell Differentiation and Is Required for Allergic Airway Inflammation and Normal IL-2 Homeostasis

Posttranscriptional control by RNA binding proteins (RBPs) of CD4⁺ T cell development is poorly understood. We previously demonstrated that the RBP HuR (elavl1) controls Th2 differentiation via increased stabilization of Gata-3, IL-4 and IL-13. HuR CD4⁺ T from transgenic mice which over-expressed HuR had significant increases in Th2 but not Th1 cytokines. IL-2 is the primary growth factor for activated T cells. When IL-2 engages its’ low affinity receptor, IL-2Rα (CD25) this results in phosphorylation of stat5 which turns off IL-2 via prdm1, which encodes the IL-2 transcriptional repressor, blimp-1. p-stat5 also opens up accessibility of the IL-4 gene to transcription, leading to Gata-3 activation, which is essential for Th2 differentiation. We hypothesized HuR regulates CD4⁺ T cell differentiation and is required for normal IL-2 homeostasis. We generated a HuR conditional KO model to study effects of HuR ablation in CD4⁺ T cell activation (distal lck-cre-ROSA HuR^fl/fl). The ROSA gene expresses YFP which is used to track HuR KO cells. We sorted CD4⁺ T cells into YFP⁺ and YFP- groups and activated them with anti-CD3/CD28. Activated YFP⁺ CD4⁺ T HuR KO cells had profound defects in Gata-3, IL-4, IL-5 and IL-13 expression and could not shut off IL-2. Up to 97% of activated T cells were unable to turn off IL-2 expression.  HuR KO T cells had defects in proliferation and JAK-STAT signaling with reduced p-stat5 and were unable to up-regulate CD25. We found significantly increased IL-2 but decreased Gata-3, prdm-1, IL-4 and CD25 transcription. We investigated whether HuR directly controlled CD25 expression. We verified HuR interacts with defined AU-rich elements (AREs) in CD25 3’UTR. CD25 mRNA stability was unchanged in HuR KO T cells but there were translational defects in recruitment of CD25 mRNA transcripts to heavy polysomes. To determine in vivo relevance, we used the ovalbumin challenge model of allergic airway inflammation. HuR KO mice had significant decreases in lung neutrophils, lymphocytes, eosinophils and IL-13. Remarkably, immunized HuR KO mice had comparable levels of total lung inflammation as un-immunized controls. We confirmed increased IL-2 and reduced CD25 phenotype in CD4⁺ T cells from immunized HuR KO mice. In summary, these data suggest that HuR plays a major role in CD4⁺ Th2 differentiation and normal IL-2 homeostasis by controlling CD25 mRNA translation. Furthermore, HuR KO in T cells completely abrogates allergen induced lung inflammation. Therefore, HuR-CD25 interactions are required for normal IL-2 homeostasis and Th2-mediated allergic airway inflammation.