Levels of a common, chronic virus (TTV) reflects the immune competence of transplant recipients

Successful organ transplantation requires careful immune suppression: enough to block the rejection of transplant while permitting host defense against infectious microbes.   Viruses that are not cleared by our immune systems, are common in healthy people, and can complicate transplantation include cytomegalovirus (CMV) and Torque teno virus (TTV), which was first described in 1997 [review].  TTV is a small (3.8 kb), single-stranded, transfusion transmitted DNA virus, representative of a highly diverse family of anelloviruses.   

The authors examined the influence of immune-suppressive drugs (e.g., tacrolimus, mycophenolate mofitil, cyclosporine) and the anti-CMV drug valgancyclovir on chronic, endogenous microbes.  From 96 heart or lung transplant recipients they collected 656 blood samples over time, some up to a year post-transplant, removed the cells, and identified remaining DNA by sequencing.  They found that 0.12% matched viral or bacterial or fungal sequences.  They validated some ‘hits’ with quantitative PCR.  Control preparations using water or bacteriophage demonstrated no relevant artifacts or contamination.  

They found that treatment with valgancyclovir reduced herpesviruses, including CMV, but dramatically increased the relative and absolute levels of anelloviruses, including TTVs (fig. 2, 3, 4).   Moreover, those patients who did not reject their transplants tended to have a greater increase in anelloviruses (Fig. 5A, shown; rejecting patients plotted in red).  The authors conclude that anellovirus levels might be used to monitor immune competence.   Focosi et al. made a related observation following autologous stem cell transplantation. 

 Cell. 2013 Nov 21;155(5):1178-87. Temporal response of the human virome to immunosuppression and antiviral therapy.  De Vlaminck I, Khush KK, Strehl C, Kohli B, Luikart H, Neff NF, Okamoto J, Snyder TM, Cornfield DN, Nicolls MR, Weill D, BernsteinD, Valantine HA, Quake SR. 

Salt develops a taste for Th17 lymphocytes

Helper T lymphocytes that make the hormone interleukin-17 (IL-17), called Th17 cells, contribute to inflammation and autoimmune diseases (review). The development of Th17 cells was known to require IL-23 but it was not known exactly why.  To gain some perspective, the authors measured gene transcripts found in Th17 cells as they develop over time from naïve mouse T lymphocytes treated with transforming growth factor-beta (TGFb) and IL-6.  They found that SGK1, an enzyme that phosphorylates proteins and has been shown to regulate sodium (Na+) transport and salt (NaCl) balance in other cells, was induced nearly 200-fold.  They emphasize that IL-23 is “critical” to the induction and maintenance of SGK1 but much of that evidence is relegated to the supplement data.  “Network analysis” with a computer program strengthened their suspicion that SGK1 is a “node” in the IL-23 signaling pathway.  

Mice without SGK1 (SGK1-knockouts, KO) have fewer Th17 cells that make less IL-17 when treated with IL-23; notably SGK1 deficiency also alters genes regulating other T cell subsets, including interferon-gamma (Ifng), Tbx21, and Gata3 (see also the previous gloss on transcription factors regulating Th17).  To test the role of SGK1, they immunized “floxed” SGK1 (conditional KO) mice with a myelin protein (MOG), which induces in some mouse strains a multiple sclerosis (MS) like disease called experimental autoimmune encephalitis (EAE).  EAE severity was significantly reduced in mice without SGK1 in Th17 cells or CD4+ helper T cells,  (fig 2a, KO score <1 normal="" vs.="">3), which corresponded with a greatly reduced number of Th17 cells in the organ targeted by this autoimmune disease, the central nervous system (CNS).  They also saw that CNS-infiltrating cells in EAE had expressed IL-17 at one time (eYFP+, fig 2e) but that expression of IL-17 by CD4 cells was lost in SGK1-KO animals (eYFP+ IL17-), suggesting that SGK1 was required to maintain expression.

That was nice but now the spice – could dietary salt modulate immunity through SGK1?    Indeed, they found that a high salt diet (HSD) accelerates the development of EAE in normal mice (fig 4e, top 2 lines, trend line offset to the left is HSD) while it does nothing to the GSK1-KO animals (lower, lines).  And connecting at least one of the dots between diet and Th17, they found that HSD also increased more than 2-fold Th17 cells and to a lesser degree interferon-gamma expressing cells in the CNS,  and the induction depended on SGK1 (fig 5f, shown, Th17 left, IFNg right; open bars SGK1-CD4-KO).  A companion paper pursued the role of dietary salt in EAE   

Nature. 2013 Apr 25;496(7446):513-7.  Induction of pathogenic TH17 cells by inducible salt-sensing kinase SGK1.  Wu C, Yosef N, Thalhamer T, Zhu C, Xiao S, Kishi Y, Regev A, Kuchroo VK.

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.