Yoshiya Mitsuhashi,1,2Akira Nakamura,1,3Shota Endo,1Kazuya Takeda,3Toshiki Yabe-Wada,3Toshihiro Nukiwa,2and
1Department of Experimental Immunology, Institute of Development,Aging and Cancer, Tohoku University, Sendai, Japan;2Department of Respiratory Medicine,
Tohoku University Graduate School of Medicine, Sendai, Japan; and3Department of Immunology, Kanazawa Medical University, Ishikawa, Japan
Plasmacytoid dendritic cells (PDCs) pro-
duce type I interferons (IFNs) in response
to viral nucleic acids to exert antiviral
immunity. However, PDCs are related to
the progress and severity of autoimmune
diseases, such as systemic lupus ery-
thematosus, because they respond to
host DNA. Therefore, the regulation of
PDC activation is critical for maintaining
adequate immune responses. Here we
show that an inhibitory major histocom-
patibility complex class I receptor, paired
immunoglobulin-like receptor B (PIR-B),
suppressed Fms-like tyrosine kinase
3 ligand-induced PDC differentiation in
BM cells, as well as Toll-like receptor
9-mediated IFN-? production by PDCs,
through the dephosphorylation of STAT1/
STAT2. In particular, PIR-B inhibited IFN-
?–mediated STAT phosphorylation, sug-
gesting that PIR-B negatively regulates
the positive feedback mechanism of
IFN-? secretion triggered by Toll-like
receptor 9. These results demonstrate a
novel regulatory role for PIR-B in PDCs.
Plasmacytoid dendritic cells (PDCs) are DC subsets specialized for
the production of type I interferons (IFN-?/?) in response to viral
infection.1,2PDCs exclusively express Toll-like receptor 7 (TLR7)
and TLR9, which recognize single-strand RNA and double-strand
DNA, respectively.1,2Stimulation of TLR7/TLR9 leads to the
production of pro-inflammatory cytokines and type I IFNs. How-
tion of type I IFNs because PDCs lacking IFN-?/? receptor barely
secrete type I IFNs, demonstrating the positive feedback mecha-
nism of type I IFN secretion initiated by TLR7/TLR9.1-3
Several receptors regulate IFN secretion by PDCs. Siglec H and
NKp44 inhibit TLR9-induced type I IFN secretion.2,4,5Moreover,
immunoglobulin (Ig)–like transcript 7 represses the production of
type I IFNs by interacting with its ligand, BM stromal cell antigen
2 (BST2).6BST2 is induced on surrounding cells; hence, the
interaction of Ig-like transcript 7 with BST2 provides a negative
feedback system for regulating the excessive production of IFNs by
PDCs. However, the involvement of other inhibitory receptors and
whether there is an autoregulatory system on PDCs remain unclear.
Paired Ig-like receptor B (PIR-B) is also an inhibitory receptor
to its phosphotyrosylated cytoplasmic domain.7PIR-B is a pair
receptor, which consists of activating-type PIR-A.Although previ-
ous studies showed that PIR-B regulated various immune cells,8it
remains unclear whether PIR-B is expressed on PDCs and whether
PIR-B affects the function of PDCs.
In the present study, we show that PIR-B can regulate not only
Fms-like tyrosine kinase 3 ligand (Flt3-L)–induced PDC differen-
tiation but also TLR9-mediated IFN-? production. In particular,
PIR-B dampens IFN-?–mediated signaling, suggesting that PIR-B
attenuates the positive feedback mechanism of IFNs secretion
triggered by TLR9.
Pirb?/?mice were generated9and backcrossed with C57BL/6 (B6) mice.
Experimental protocols were approved by theAnimal Studies Committee at
the Institute of Development, Aging, and Cancer, Tohoku University, or
Kanazawa Medical University.
Flow cytometry and immunoblot assay
Flow cytometry and immunoblot assays were done according to standard
methods. Antibodies for flow cytometry were obtained from BioLegend.
Antibodies for immunoblot assay were obtained from Cell Signaling
Technology and Sigma-Aldrich.
Results and discussion
We first investigated the population of splenic PDCs (PDCA-1?
B220?cells) in B6 and Pirb?/?mice. The population of splenic
PDCs was comparable between B6 and Pirb?/?mice (Figure 1A),
suggesting that PIR-B is not implicated in the development of
PDCs in vivo.
We next examined PIR-B expression on splenic PDCs. Pirb?/?
PDCs showed a low level of PIR-A expression, demonstrating a
dominant expression of PIR-B on PDCs (Figure 1B). PDCs are also
derived from Flt3 ligand (Flt3-L)–induced BMDCs.1Thus, we
induced BMDCs from BM cells for 8 days under Flt3-Lcontaining
medium, and PDCs were purified from BMDCs. In contrast to in
vivo results, the total cell numbers of BMDCs in Pirb?/?mice were
increased (Figure 1C). The ratio of PDCs in BMDCs was also
augmented in Pirb?/?mice, suggesting that PIR-B repressed
FLt3-L–mediated signaling (Figure 1D). Conversely, PIR-B was
Submitted March 21, 2012; accepted August 14, 2012. Prepublished online as
Blood First Edition paper, September 4, 2012; DOI 10.1182/blood-2012-
The online version of this article contains a data supplement.
The publication costs of this article were defrayed in part by page charge
payment. Therefore, and solely to indicate this fact, this article is hereby
marked ‘‘advertisement’’ in accordance with 18 USC section 1734.
© 2012 by TheAmerican Society of Hematology
3256BLOOD, 18 OCTOBER 2012?VOLUME 120, NUMBER 16
also dominantly expressed on FLt3-L–induced PDCs (Figure 1E).
The expression level of Flt3 and MHC class II was comparable
between B6 and Pirb?/?PDCs (Figure 1E).
To elucidate how PIR-B regulates PDC differentiation induced
by Flt3-L, we checked PDC progenitor fractions.10The population
of lineage?Flt3?BM cells between B6 and Pirb?/?mice was
comparable (Figure 1F). Therefore, we investigated the PDC
subset at day 4 and day 6 after administration with Flt3-L. The
population and numbers of Pirb?/?B220?CD11c?cells were
almost the same as that of B6 B220?CD11c?cells (Figure 1G;
supplemental Figure 1A, available on the Blood Web site; see the
Supplemental Materials link at the top of the online article). B6 and
Pirb?/?B220?CD11c?cells also expressed the same level of Flt3
(Figure 1H). Conversely, the expression of PIR-B on B6
B220?CD11c?cells was remarkably up-regulated at day 6 (supple-
mental Figure 1B), suggesting that PIR-B regulates PDC differen-
tiation from day 6 to day 8. Therefore, we tested Flt3 signaling in
B220?CD11c?cells at day 6. The phosphorylation of Shc, Erk1/2,
and STAT3 was enhanced in Pirb?/?B220?CD11c?cells (Figure
1I), demonstrating that PIR-B suppressed Flt3-L–mediated signal-
ing. However, the depletion of PIR-B did not affect PDC develop-
ment in vivo (Figure 1A). GM-CSF suppresses Flt3-L–dependent
Figure 1. PIR-B represses the development of Flt3-L–
induced PDCs. (A) Splenocytes isolated from 8-week-
old mice are stained with anti–PDCA-1 and CD11c. The
percentages of PDCA-1?CD11c?PDCs (circled). Data
are mean ? SEM (n ? 4 mice per group). (B) Flow
cytometric analysis of PIR-A/B expression on B6 and
Pirb?/?splenic PDCs. Black line indicates B6 PDCs; and
gray line, Pirb?/?PDCs. Occupied gray area represents
isotype control. Mean fluorescent intensities of PIR-A/B
expression. (C) BM cells from B6 and Pirb?/?mice are
incubated with Flt3-L (150 ng/mL) for 8 days. The total
cell numbers of BM cells. F represents B6 BM cells; and
E, Pirb?/?BM cells. Data are mean ? SEM (n ? 6 mice
per group). (D) The percentages of Flt3-L–induced PDCs
(circled). Data are mean ? SEM (n ? 3 mice per group).
(E) Flow cytometric analysis of PIR-A/B, Flt3, and I-A/E
expression on B6 and Pirb?/?Flt3-L–induced PDCs.
Mean fluorescent intensities. (F) The progenitor fraction
of PDCs (Flt3?lineage?) in B6 and Pirb?/?BM cells. Data
are mean ? SEM (n ? 3 mice per group). (G) The
percentages of B220?CD11c?BM cells at 4 and 6 days
(n ? 3 mice per group). (H) Flow cytometric analysis of
Flt3 expression on day 4 and 6 B220?CD11c?BM cells.
Mean fluorescent intensities. (I) B220?CD11c?BM cells
at 6 days after Flt3-L administration were cultured in
Flt3-L–free conditions for 3 hours to reduce endogenous
signaling activity and were then stimulated with Flt3-L
(300 ng/mL). Immunoblot analysis of phospho-STAT3
(pSTAT3), STAT3, pErk1/2, Erk1/2, pShc, and Shc. All
results are representative of 3 separate experiments. All
statistical analyses were performed using Student t test:
*P ? .05, **P ? .01.
REGULATION OF PDCs BY PIR-B 3257 BLOOD, 18 OCTOBER 2012?VOLUME 120, NUMBER 16
PDC development.11Furthermore, the population of immediate
PDC precursors, which have plasticity to conventional DCs under
mental Figure 2). The in vivo development of PDCs in Pirb?/?
mice might be blocked by GM-CSF.
To examine the role of PIR-B in TLR9-mediated activation, we
confirmed the same level of cytoplasmic TLR9 expression in B6
and Pirb?/?PDCs (Figure 2A). Thus, we measured the amount of
IFN-? produced by B6 and Pirb?/?PDCs after CpG-Astimulation.
IFN-? produced by Pirb?/?PDCs was higher than that by B6
PDCs (Figure 2B), indicating that PIR-B suppressed TLR9-
mediated activation. There are 2 signaling pathways activated by
TLR9 stimulation in PDCs: one leads to the production of
pro-inflammatory cytokines and the other to the production of type
I IFNs.13After CpG-Astimulation, PIR-B phosphorylation and the
association with SHP-1 were increased in B6 PDCs (Figure 2C).
We investigated pro-inflammatory cytokine signaling pathways,
such as ERK and p38. However, the phosphorylation of ERK and
p38 was slightly reduced in Pirb?/?PDCs (Figure 2D). Therefore,
we examined the alternate pathway that leads to IFN production.
However, Pirb?/?PDCs also showed reduced I?B kinase-?
phosphorylation (Figure 2E). In B-1 cells, PIR-B represses TLR9-
mediated activation through Bruton tyrosine kinase (Btk) dephos-
phorylation.14However, the Btk phosphorylation barely increased
in B6 and Pirb?/?PDCs (Figure 2F). Furthermore, the IL-12
production was reduced in Pirb?/?PDCs (supplemental Figure 3).
These results indicated that PIR-B did not repress TLR9-mediated
signaling in PDCs.
Conversely, the robust production of type I IFN by PDCs
requires a positive feedback mechanism mediated by autocrine
type I IFNs.1-3Thus, to elucidate whether PIR-B affects type I
IFN-mediated signaling, we examined the IFN-?/? receptor down-
stream signaling molecules, STAT1/STAT2, which activate the
transcription of IFN production. We found that phosphorylation of
STAT1/STAT2 increased 2 hours after CpG-A stimulation (Figure
2G), suggesting that autocrine type I IFNs activate STATphosphor-
ylation. Moreover, Pirb?/?PDCs showed enhanced phosphoryla-
tion of STAT1/STAT2 compared with B6 PDCs. Therefore, we
stimulated PDCs with IFN-?. PIR-B phosphorylation and the
association with SHP-1 were increased in B6 PDCs (Figure 2H).
Figure 2. PIR-B negatively regulates the positive
feedback mechanism of IFN-? secretion triggered by
TLR9. (A) Flow cytometric analysis of cytoplasmic TLR9
expression on B6 and Pirb?/?Flt3-L–induced PDCs.
Mean fluorescent intensities. (B) B6 and Pirb?/?PDCs
are incubated with CpG-A (OD2216) for 24 hours at the
indicated concentration. The amounts of IFN-? are mea-
sured by ELISA assay. Data are mean ? SEM (n ? 4).
The results are representative of 3 separate experi-
ments. Statistical analyses were performed using Stu-
dent t test: *P ? .05. (C-G) Immunoblot analysis of B6
and Pirb?/?PDCs stimulated with CpG-A (3 ?g/mL).
(C) Immunoblots of PIR-B phosphotyrosine (4G10),
SHP-1, and PIR-B after the precipitation with anti–PIR-
A/B. (D) Immunoblots of pErk1/2, Erk, pp38, and p38.
(E) Immunoblots of pI?B kinase ?- and ?-actin. (F) Immu-
noblots of pBtk and Btk. (G) Immunoblots of pSTAT1,
STAT1, pSTAT2, and STAT2. (H-I) Immunoblot analysis
of B6 and Pirb?/?PDCs stimulated with IFN-? (100 U/
mL). (H) Immunoblots of PIR-B phosphotyrosine (4G10),
SHP-1, and PIR-B after the precipitation with anti–PIR-
A/B. (I) Immunoblots of pSTAT1, STAT1, pSTAT2, and
STAT2. The immunoblot results are representative of
3 separate experiments.
3258 MITSUHASHI et al BLOOD, 18 OCTOBER 2012?VOLUME 120, NUMBER 16
The phosphorylation status of STAT2 (but not STAT1) was Download full-text
increased in Pirb?/?PDCs (Figure 2I). These findings suggested
that PIR-B repressed the type I IFNs secreted by the positive
feedback mechanism. PIR-B functions as an autoregulatory system
for preventing excessive secretion of type I IFNs in PDCs, different
from the regulatory system of the Ig-like transcript 7-BST2
interaction. Collectively, PIR-B regulates cytokines-mediated sig-
naling in PDCs, suggesting that PIR-B could be a therapeutic target
for autoimmune disorders.
This work was supported by the Core Research for Evolutional
Science and Technology Program of the Japan Science and
Technology Agency (T.T.), the Ministry of Education, Culture,
Sports, Science and Technology of Japan (grant-in-aid; A.N. and
T.T.), and the Global Century Center of Excellence Program
“Innovative Therapeutic Development Toward the Conquest of
Signal Transduction Diseases with Network Medicine” (T.T.).
Contribution:Y.M.,A.N., and T.T. designed research and wrote the
manuscript; Y.M., A.N., S.E., K.T., and T.Y.-W. performed the
experiments; and T.N. supervised the research.
Conflict-of-interest disclosure: The authors declare no compet-
ing financial interests.
Correspondence: Toshiyuki Takai, Department of Experimental
Immunology, Institute of Development,Aging and Cancer, Tohoku
University, Seiryo 4-1, Aoba-ku, Sendai 980-8575, Japan; e-mail:
firstname.lastname@example.org; and Akira Nakamura, Department of
Immunology, Kanazawa Medical University, Daigaku 1-1, Uchi-
nada, Ishikawa 920-0293, Japan; e-mail: aki-n@kanazawa-
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