in vivo

esult in clonal expansion and IFN-γ production
by antigen-specific CD8 + T cells. Consistent
with the previous in vitro findings, TLR3 and
TLR7 were both required for the clonal T-cell
expansion. Notably, TLR3 was more critical
than TLR7, for generating IFN-γ-producing
CD8 + T cells. Interestingly, type I IFNs, which are
thought to play a major role in the generation
of CD8 + T-cell responses, were dispensable for
responses induced by poly(A:U). Our results
demonstrate that poly(A:U) is an in vivo and in
vitro immunoadjuvant functioning mainly through
TLR3 and TLR7. The next important issue is to
clarify the mechanisms by which TLR3 signaling
induces CD8 + T-cell responses
The regulation of innate immune responses
A nuclear ubiquitin E3 ligase, PDLIM2/
SLIM, interacts with STAT4, the transcription
factor essential for IL-12-mediated T-helper 1
(Th1) cell differentiation (Tanaka et al. Immunity
22, 729-736, 2005). PDLIM2 contains a PDZ
domain at the N-terminus and a LIM domain
at the C-terminus and belongs to a large family
of LIM proteins. In CD4 + T cells, PDLIM2 can
interact with STAT proteins in the nucleus
and promote their polyubiquitination and
subsequent proteasomal degradation. Thus,
PDLIM2 negatively regulates STAT-dependent
signaling. Consistent with these observations,
Figure 1
Molecular mechanisms of
poly(A:U)-induced adjuvant
effects.
Murine BM DCs can be
generated in the presence of
Flt3L or GM-CSF. Flt3L-induced
BM DCs contain both pDC and
cDC, the latter of which can be
further divided into two subsets.
By contrast, GM-CSF-induced
BM DCs contain only cDC.
Notably, all of these DC subsets
are distinct in the expression
pattern of and responsiveness
to TLRs and cytosolic sensors.
poly(A:U) fails to activate
cytosolic sensors but can
stimulate DCs through TLR3 or
TLR7.
Th1 cells from PDLIM2-deficient mice have
increased levels of STAT protein and enhanced
IFN-γ production
Activation of the NF-κB transcription factor
in innate immune cells is also tightly regulated to
prevent excessive inflammatory responses. How
NF-κB activation is terminated, however, is not
fully understood. After stimulation with various
TLR ligands, NF-κB activation is first upregulated,
then downregulated and terminated. During the
downregulating phase, an NF-kB subunit, p65,
is ubiquitinated and degraded in the nucleus.
Expression of PDLIM2 suppressed NF-κB
activity. PDLIM2 bound to p65 and promoted
p65 polyubiquitination in the nucleus. In addition,
PDLIM2 targeted polyubiquitinated p65 to
discrete intranuclear compartments where it was
degraded in a proteasome-dependent manner.
This intranuclear translocation of p65 depended
on the PDZ domain of PDLIM2. Furthermore,
PDLIM2-deficiency led to defective p65
ubiquitination and an increase in nuclear p65
levels. Consistent with these findings, PDLIM2-
deficient DCs showed enhanced production of
proinflammatory cytokines in response to TLR4
or TLR9 signaling and the mutant mice were
hypersensitive to endotoxin shock. Our findings
define a pathway by which PDLIM2 terminates
NF-κB activation through nuclear sequestration
and subsequent degradation.
Recent publications
K. Hoshino, T. Sugiyama, M.
Matsumoto, T. Tanaka, M. Saito,
H. Hemmi, O. Ohara, S. Akira,
T. Kaisho. 2006. IκB kinase-α is
critical for interferon-α production
induced by Toll-like receptors
7 and 9. Nature. 440, 949-953
(2006).
D. Kinoshita, F. Hirota, T. Kaisho,
M. Kasai, K. Izumi, Y. Bando, Y.
Mouri, A. Matsushima, S. Niki, H.
Han, K. Oshikawa, N. Kuroda, M.
Maegawa, M. Irahara, K. Takeda,
S. Akira, M. Matsumoto. 2006.
Essential role of IκB kinase α in
thymic organogenesis required for
the establishment of self-tolerance.
J. Immunol. 176, 3995-4002
(2006).
T. Kaisho, S. Akira. 2006. TLR
function and signaling. J. Allergy
Clin. Immunol. 117, 979-987
(2006).
T. Tanaka, M. J. Grusby and
T. Kaisho. PDLIM2-mediated
termination of transcription factor
NF-κB activation by intranuclear
sequestration and degradation of
the p65 subunit. Nature Immunol.
8, 584-591 (2007).
T. Sugiyama, K. Hoshino, M. Saito,
T. Yano, I. Sasaki, C. Yamazaki ,
S. Akira and T. Kaisho.
Immunoadjubant effects of
polyadenylic-polyuridylic acids
through TLR3 and TLR7. Int.
Immunol. 20, 1-9 (2008).
Figure 2
PDLIM2-mediated
regulation of NF-κB activity.
PDLIM2 can bind and
polyubiquitinate the p65
subunit of NF-κB in the
nucleus. The p65 is then
degraded in specialized
nuclear regions in a
proteasome-dependent
manner.
49