Sunday, April 21, 2013

Recipe for Developing Th17 cells

Thymus-dependent “T” lymphocytes develop into several effector and regulatory lineages, including the well-characterized regulatory “helper” T (Th) cells that express the cellular differentiation marker 4 (CD4+) and CD8+ cytotoxic T lymphocytes (CTL, or Tc) that kill virus-infected cells. The CD4+ Th lineages further differentiate into Th1, Th2, and Treg cells that help protect against intracellular microbes, or helminthes, or specifically regulate immune responses, as well as Th17 cells, so-called because they make the interleukin-17 (IL17) that is required for protecting the mucosa against infection by bacteria and fungi.  

Development of cell lineages is controlled externally by cytokines and internally by “master” transcription factors.  RORgt (retinoic-acid-receptor-related orphan receptors gamma t) is expressed by Th17 cells and forced expression of RORgt gene in naïve CD4+ T cells (Th0) makes them express some genes characteristic of Th17 cells such as the IL-23 receptor and the chemokine receptor CCR5 but not the full range of Th17 products, which requires other TFs including STAT3, IRF4, BATF, and IkappaBzeta.  Other TFs may replace these for inducing some Th17 genes.  The myriad of TFs required for more or less full Th17 function led these investigators to try to sort out how they work together. 
The authors first looked where these implicated TFs bind on the genome of Th0 cells treated with Th17-inducing cytokines using chromosome immune-precipitation (ChIP).  They then compared genes that are transcribed, measured using RNA seq, in the absence of specific TFs, reduced using siRNA, to “build a network model for Th17 cells”.  

They propose that TFs BATF and IRF4 bind cooperatively and open chromosomes to STAT3, which drives transcription of many genes including the lineage-specifying TF RORgt.  They also identify several putative new Th17 regulators, including the AP-1 family member Fosl2, which they belive is a key TF for Th17 development.  They derive many complicated, colorful figures.  A largely understandable, intriguing single figure is 5D, copied here, which shows that a block of Th17-related genes is increased (red) or decreased (blue) “log2 fold” (NB the genes, named on the right, are NOT the same) in Th17 cells but not other T cell subsets (Th1, Th2) treated with siRNA suppressing Satb1 (a “chromatin organizer"), Bcl11b (a zinc finger TF), Jmjd3 (a histone demethylase), and the old familiar RorC (encoding RORg).   But why no siRNA for Fosl2? 

A validated regulatory network for th17 cell specification.  Ciofani M, Madar A, Galan C, Sellars M, Mace K, Pauli F, Agarwal A, Huang W, Parkurst CN, Muratet M, Newberry KM, Meadows S, Greenfield A, Yang Y, Jain P, Kirigin FK, Birchmeier C, Wagner EF, Murphy KM, Myers RM, Bonneau R, Littman DR.   Cell. 2012 Oct 12;151(2):289-303.

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