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Cellular & Molecular Immunology (2013) 10, 2–3
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RESEARCH HIGHLIGHT
One way to pathogenesis, many ways to homeostasis
Linrong Lu1,2 and Jianli Wang1
Cellular & Molecular Immunology (2013) 10, 2–3; doi:10.1038/cmi.2012.51; published online 29 October 2012
C
D4 T cells play central roles in both
immune response and regulation.
They can either serve as the mediators of
adaptive immune response against infections,
or act as gate keepers to maintain immune
homeostasis. Upon activation by antigens,
naive T cells proliferate and differentiate into
different T helper (Th) cells with distinct
effector functions. Th1 cells, induced by transcription factor T-bet and produce interferonc, are crucial for clearing intracellular pathogens. In contrast, IL-4 expressing Th2 cells
driven by GATA-3 are responsible for the
clearing of extracellular pathogens. Th17 cells,
an additional subset of T cells expressing transcription factor RORct and cytokine IL17, are
considered as the main drivers of autoimmune
tissue injury. On the contrary, CD41FoxP31
regulatory T (Treg) cells, which can be generated either in the thymus or in the peripheral
in respond to chronic challenges, are known to
play key roles in maintaining immunological
homeostasis and the control of autoimmune
deviation or diseases. In a recent issue of
Nature Immunology, two independent studies
from Harvard Medical School described the
transcriptional signature of effector Th17 and
Treg cells and unveiled very different mechanisms in instructing these two cell types.1,2
Th17 cells are characterized by their capacities to produce IL-17A, IL-17F, IL-21 and
IL-22. In vivo, these cells are present at the site
of tissue inflammation in many autoimmune
diseases and are thought to be the critical
drivers of autoimmune tissue inflammation
1
Institute of Immunology, Zhejiang University School of
Medicine, Hangzhou, China and 2Program in Molecular
and Cellular Biology, Zhejiang University School of
Medicine, Hangzhou, China
Correspondence: Dr LR Lu, Institute of Immunology,
Zhejiang University School of Medicine, Hangzhou,
China.
E-mail: [email protected]
and Dr JL Wang, Institute of Immunology, Zhejiang
University School of Medicine, Hangzhou, China.
E-mail: [email protected]
Received: 21 September 2012; accepted: 24
September 2012
and damage. In vitro, IL-17 expressing cells
can be differentiated by a combination of
the cytokines transforming growth factor
(TGF)-b1 and IL-6. However, Th17 cells
induced by TGF-b1 and IL-6 in vitro do not
readily induce autoimmune inflammation
and need additional exposure to another
cytokine IL-23 to become pathogenic.3
The analysis of IL-23R2/2 mice also
revealed the essential role of the IL-23 signal
in Th17 cell-mediated pathogenesis, since
decreased Th17 function and reduced susceptibility to experimental autoimmune
encephalomyelitis (EAE) were observed in
those mice.4 However, how IL-23 drives
pathogenic Th17 cells is yet to be clarified.
In this issue of Nature Immunology, Lee et
al., from Vijay K Kuchroo’s laboratory, identified TGF-b3 as a key downstream factor of
IL-23 responsible for the driving of pathogenic Th17 cells.
By using detailed microarray analysis of the
gene expression in Th17 cells induced under
different conditions, the authors first found
that TGF-b3 was substantially induced in
Th17 cells and this induction depended on
the presence of the IL-23 signal, suggesting
the role of TGF-b3 in IL-23 induced pathogenic differentiation of Th17 cells. This hypothesis was later proven by the experiments
showing that Th17 cells could also be induced
by using IL-6 plus TGF-b3 instead of TGF-b1.
More importantly, TGF-b3-induced Th17
cells were as potent as Th17 cells induced by
TGF-b1, IL6 and IL-23 for inducing EAE.
Finally, the authors performed additional
microarray analysis on Th17 cells induced
by either TGF-b11 IL-61IL-23 or TGFb31IL-6 and defined the transcriptional signature of pathogenic Th17 cells, represented
by 233 genes differentially expressed in pathogenic Th17 cells compared to non-pathogenic
Th17 cells. Among which, 23 of them are closely relevant to the pathogenic functions of
Th17 cells, including the elevated expression
of distinct chemokines and cytokines (Cxcl3,
Ccl4, Ccl5, Ccl3, Csf2, Il3, Il22 and Casp1),
transcription factors Tbx21 and STAT4 and
several effector molecules, such as Gzmb, Lag2
and Lglas. Pathogenic Th17 cells also downregulate the expression of IL-10, IL9, IL1Rn and
Ikzf3, which are upregulated in non-pathogenic
Th17 cells. Upregulation of Tbx1 (which
encodes transcription factor T-bet) in pathogenic Th17 cells is also well consistent with
the essential role of T-bet in the pathogenesis
of Th17 cells. Interestingly, the defect of pathogenic Th17 cells in Tbx12/2 mice could be
overcome by the treatment of TGF-b3, suggesting that T-bet might regulate the endogenous
expression of TGF-b3. In summary, this study
revealed the essential function of IL-23/TGFb-3
axis in driving pathogenic Th17 cells, as judged
by their capacity to induce both the ‘pathogenic
signature’ and the actual pathogenic functions
of these cells.
In the same issue of Nature Immunology, Fu
et al., from Diane Mathis and Christophe
Benoist’s laboratory, used a large scale transcriptome profiling analysis approach to study
the signature of CD41FoxP31 Treg cells. The
development of Treg cells in the thymus or
induction of Treg cells in vitro is characterized
and relies on the expression of transcription
factor FoxP3. However, the transduction of
FoxP3 or its induction by TGF-b in vitro is
not sufficient to elicit the full Treg cell signature. In fact, FoxP3 has been reported to
interact with many different transcription factors including NFAT, Runx1, Eos, Stat3, IRF4
and so forth, many of which are indeed found
to be important for the development or full
functions of FoxP31 Treg cells. However, we
still don’t know how the contributions of
these different factors are orchestrated in the
process of Treg cells induction.
The authors started with the analysis of a
‘Treg signature’ (composed of 603 target
genes differentially expressed in canonical
Treg cells) in 129 gene expression profiles
from various CD41 T cells including cells
from mice which are deficient in the expression
Research Highlight
3
Th17
IL-23
T-bet?
Non-pathogenic
TGF-b3
Th17
Pathogenic
Signature
Pathogenic
FoxP3 + Eos
FoxP3 + IRF4
NaÏve T
FoxP3 + GATA-1
Treg
Treg
Signature
FoxP3 + Satb1
Locked in
FoxP3 + Lef1
FoxP3 + X
Figure 1 Driving pathogenic Th17 cells and stable Treg cells. (a) IL-23/TGF-b3 axis in driving pathogenic
Th17 cells: IL-23 is essential for the stabilization of Th17 cells and their ability to induce autoimmune tissue
inflammation through the induction of TGF-b3 expression in Th17 cells, which might be mediated by the
activation of T-bet; (b) The multiple redundancy in the Treg cell induction: induction of stable Treg cells
expressing full Treg signature required the expression of FoxP3 together with any one of the other factors,
including Eos, IRF4, GATA-1 lef1 and Sabt1, etc. TGF, transforming growth factor; Treg, regulatory T.
of the transcription factors that interact with
FoxP3. By using the context likelihood of relatedness algorithm, the authors were able to
align the key transcription factors according
to their contribution ‘scores’ to the expression
of this Treg signature. As expected, this analysis
identified FoxP3 as the top predicted regulator
with the highest score. It also revealed a number of other transcriptional regulators of the
Treg signature, including Eos, Helios, Lef1
and GATA-1, some were shown previously to
be associated with Treg cell function.5,6 The
actual contributions of four transcription factors, Eos, GATA-1, Xbp1 and Helios, were then
verified by direct expression profiling of Treg
cells from the knockout mice of these genes.
Surprisingly, the Treg signatures in each of
these mutated mice were not altered significantly compared to wild-type cells. This finding was intriguing but consistent with the
normal Treg cell numbers and functions
in these mice as shown previously.7–9
However, this observation did not exclude
the contributions of these factors to the stabilization of Treg cells. Indeed, when the authors
overexpressed these factors together with
FoxP3, they found that overexpression of
FoxP3 alone only led to a small fraction of
Treg signature genes, while the co-expression
of any of these factors with FoxP3 was capable
of inducing the full Treg signature robustly.
Two inputs were thus needed to work
synergistically to ‘lock in’ the Treg signature and the establishment of the stable
Treg cell state. The ‘two key’ control system
diminishes the risk of the erroneous activation
of Treg cells under the circumstances when
FoxP3 is transiently induced. More interestingly, as judged by the expression pattern of
the ‘Treg signature’, five of the factors (including Eos, IRF4, GATA-1, Lef1 and Satb1) regulated the same set of genes together with
FoxP3, which revealed the multiple redundancy of this lock-in process. It is worth mentioning that, although this study showed that
the synergistic effect can be mediated by five
different factors, the list might still be incomplete. This redundancy not only ensures additional stability, but also allows several different
physiological pathways to arrive at the same
state, which may be relevant to the different
thymic and extra-thymic context of Treg cell
differentiation. Different conditions might
each induce one or another cofactor to enable
a ‘lock-in’ of the Treg cell transcriptional network.
Taken together, these two papers described
the essential factors in driving or maintaining
either pathogenic Th17 cells or Treg cells using
the transcriptional signature as a marker. Both
pathogenic Th17 cells and Treg cells are tightly
controlled, since both Th17 and Treg cells
require at least two different factors to reach
a fully functional state (either to be pathogenic
for Th17 cells or to be ‘locked in’ for stable
Treg cells). Although a definitive surface marker is still not available, pathogenic Th17 cells
and ‘locked-in’ Treg cells can now be defined
by their transcription signature as described in
both papers. Extensive analysis of these ‘expression signatures’ and associated experimental
evidence in these two reports also leads to our
understanding of the process or architecture of
the differentiation of these cells, which are very
different. Pathogenic Th17 cells are driven
strictly by the IL-23/TGF-b3 axis. In contrast,
Treg cells have multiple choices of many different factors or pathways (including, but not
limited to, the five factors identified by Fu et al.)
to fulfill their differentiation (Figure 1). These
different regulation modes well represent the
nature of the immune system. The immune
system is doing its best to maintain homeostasis
and prevent autoimmunity, which could be
achieved by inducing Treg cells through many
different ways under different circumstances or
at different locations. For each deliberate
immune response against infection, when an
effective clearing of pathogens is necessary, the
immune system also manages to keep the reaction well controlled to prevent excessive activation and self-destruction, which makes it
crucial to keep the pathogenesis pathways well
constrained in multiple steps.
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5
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Cellular & Molecular Immunology