genomewide characterization of host-pathogen interactions by ...

Maren Depke
Discussion and Conclusions
associated groups of genes, of which few were regulated at the early and several at the later time
point of analysis. Examples are given with leukemia inhibitory factor LIF (induced at both time
points) and its receptor (induced after 6.5 h), ligand CD274 (PD-L1, induced at both time points)
and PD-L2 (induced after 6.5 h), several interferon regulated genes at both time points or
additionally at the 6.5 h time point, and the histone genes, of which one was repressed at the
2.5 h time point and further 17 were added at the 6.5 h time point.
Furthermore, indoleamine 2,3-dioxygenase 1 (IDO1) belonged to the genes with the highest
induction factors at the 6.5 h time point. The gene codes for a key molecule in
immunomodulation, microbial growth control, and pathogen immune escape (Puccetti 2007,
Zelante et al. 2009). IDO is known to be induced by interferons (Carlin et al. 1989,
Taylor MW/Feng 1991) e. g. during infection and inflammation, but also in conditions like stress,
which result in a transient increase of proinflammatory cytokines (Kiank et al. 2010). The enzyme
catalyzes the reaction of tryptophan to formylkynurenine. This is the first rate-limiting step of the
so-called “kynurenine pathway”, which leads to NAD + biosynthesis or to the complete oxidative
degradation of kynurenine (King/Thomas 2007, Xie et al. 2002). One of the main effects of
induced IDO activity is the depletion of tryptophan, which is discussed to be a mechanism to
inhibit microbial growth (Pfefferkorn 1984, Byrne et al. 1986, Müller et al. 2009). But this
mechanism can be counteracted by bacteria, which might induce their tryptophan biosynthesis
or which developed own counter-regulatory mechanisms. An example is Chlamydophila psittaci
which owns adapted tryptophan biosynthesis genes that enable the bacterium to produce
tryptophan from kynurenine taken from their host’s metabolism (Xie et al. 2002).
During metabolism of tryptophan, several neuro- and immune-active substances are
produced, of which some additionally exhibit toxic features and thus might act antimicrobially.
The intermediates of tryptophan and kynurenine degradation, 3-hydroxykynurenine and α-
picolinic acid, were reported to inhibit growth of S. aureus and other bacterial species in a murine
transplantation model even in the presence of tryptophan (Saito et al. 2008, Narui et al. 2009).
The most prominent characteristic of IDO is the induction of an anti-inflammatory bias after a
proinflammatory stimulus. This effect aims to limit inflammation and the accompanying damage
to the host tissue. It has to be considered that the experimental setting in this study only
included one single cell type, the bronchial epithelial cell line S9. Thus, interpretation of an antiinflammatory
effect assumes that bronchial epithelial cells might also in vivo induce IDO, e. g.
during staphylococcal pneumonia. Tryptophan degradation intermediates were shown to inhibit
proliferation of CD4 + and CD8 + T cells and of NK cells (Frumento et al. 2002). Other experiments
supported a proposed mechanism of inhibition of T cell proliferation, which was triggered by the
low concentration of tryptophan resulting from IDO activity and mediated by GCN2 kinase
(EIF2AK4, eukaryotic translation initiation factor 2 alpha kinase 4), a sensor for uncharged tRNA
(Munn et al. 2005). The contradiction that tryptophan depletion leads to an antimicrobial effect
on the one hand and to an inhibition of T cell mediated immune response on the other hand was
resolved by Müller and coworkers, who determined the concentration limit necessary for the two
effects. They observed that bacterial inhibition (50 % inhibitory concentration of 1.9 µM) took
already place at a higher tryptophan concentration whereas tryptophan had to be depleted even
further (50 % inhibitory concentration of 0.1 µM) to achieve T cell inhibition (Müller et al. 2009).
Besides the described aspects, in an in vivo infection situation dendritic cells (DC) will contribute
to the immune response. Further IDO-mediated immune-modulation mechanisms involve DC
mediated T cell tolerance (Popov/Schultze 2008). The induction of IDO in epithelial cells near
192