UCSF

Helper T cells play a critical role in the maintenance and coordination of a healthy immune system. Upon stimulation, naïve helper T cells transition into functional, effector cells that proliferate, migrate, and actively secrete cytokines to help direct an immune response. Importantly, programmed differentiation of helper T cells into unique subsets of effector cells allows for functionally different responses to different immune challenges. The actual process of helper T cell polarization involves many regulated changes in gene expression and represents both an important aspect of immune regulation and a useful system for studying cell differentiation in general.

MicroRNA (miRNA)-deficient helper T cells exhibit abnormal differentiation, cytokine production and decreased proliferation. However, the contributions of individual miRNAs to this phenotype remain poorly understood. We developed and utilized a screening strategy to assay miRNA function in primary T cells and identified individual miRNAs that regulate the cytokine production and proliferation of these cells.

We first focused on interferon (IFN)-gamma; production and found that microRNA-29 (miR-29) largely corrected the aberrant IFN-gamma; expression of miRNA-deficient cells. Repression of IFN-gamma; by miR-29 involved direct targeting of both T-bet and Eomes, two transcription factors known to induce IFN-gamma; production. These two transcription factors were elevated in miRNA-deficient cells and were also upregulated following miR-29 inhibition in wildtype cells. These results demonstrate that miR-29 regulates helper T cell differentiation by repressing multiple target genes, including at least two that are independently capable of inducing the Th1 gene expression program. Additional analyses of miRNA-mediated effects on other cytokines, including TNF, IL-4, and IL-13, revealed an ability of individual miRNAs to significantly influence the production of these cytokines as well. Notably, we found that miR-29 can promote TNF expression through effects that are dependent on the AU-rich element in the TNF 3'UTR. Taken together, these studies demonstrate the important role of individual miRNAs in cytokine regulation and raise intriguing possibilities for better understanding the gene expression networks that underlie immunity in health and disease.