Mapping a Th17 network: microRNA and Polycomb in cell fate specification

Host detection of foreign pathogens can trigger both innate and adaptive immune responses; CD4+ T helper (Th) cells play a critical role in the latter. Upon encountering its cognate antigen and receiving appropriate co-stimulatory signals, a naïve CD4+ Th cell can differentiate into a variety of effector subsets that include, but are not limited to, CD4+ Th1, Th2, Th17, Tfh, and iTregs. This dissertation focuses on the regulatory circuitry that enables the differentiation of distinct CD4+ Th cell lineages; concentrating on the role of microRNA (miRNA) in this T cell fate decision. Experimentally, in vitro polarization of primary murine CD4+ Th cells by addition of exogenous cytokines is used to decipher the molecular and signaling events that control the differentiation process of each subset. Examination of Th1, Th2, and Th17 cells deficient in Signal Transducers and Activators of Transcription (STAT) signaling, reveals the critical function of these transcription factors in inducing expression of non-coding RNA. Focusing on the CD4+ Th17 cell subset, a key role for STAT3 is identified in regulating the expression of miR-29 and miR-155. We find that miR-29 expression in Th17 cells prevents aberrant cytokine expression, and this miRNA has a further role in the in vivo generation of pathogenic Th17 cells during autoimmune disease. Moreover, we show that miR-155 promotes Th17 cytokine expression by altering the chromatin state through the inhibition of Jarid2 expression. In the context of embryonic stem cells, Jarid2 recruits the Polycomb Repressive Complex 2 (PRC2) to epigenetically silence key developmental genes by catalyzing the trimethylation of H3K27. We find that high expression of miR-155 in Th17 cells alleviates Jarid2-PRC2 inhibition of genes encoding Th17 signature cytokines. Thus, miR-155 targeting of Jarid2 favors a more open chromatin state at both cytokine loci and genes promoting Th17 cell development. Through collaboration with the National Center for Advancing Translational Sciences (NCATS), the translational aspects of these research questions were investigated. MiR-155 and Th17 cells are highly expressed in many autoimmune diseases and Th17 cells have a detrimental role in such diseases. By examining the molecular networks involving miRNAs in CD4+ Th cell development, our goal is to find new drugs that can target mir-155 for therapeutic purposes and potentially alleviate disease progression