JOURNAL OF VIROLOGY, Jan. 2010, p. 822–832
0022-538X/10/$12.00 doi:10.1128/JVI.01339-09
Copyright © 2010, American Society for Microbiology. All Rights Reserved.

Vol. 84, No. 2

Extrachromosomal Histone H2B Mediates Innate Antiviral Immune
Responses Induced by Intracellular Double-Stranded DNA䌤†
Kouji Kobiyama,1,4‡ Fumihiko Takeshita,1‡* Nao Jounai,1 Asako Sakaue-Sawano,2 Atsushi Miyawaki,2Ken J. Ishii,3,4 Taro Kawai,3 Shin Sasaki,1 Hisashi Hirano,5 Norihisa Ishii,6
Kenji Okuda,1 and Koichi Suzuki6‡*

Received 29 June 2009/Accepted 2 November 2009

Fragments of double-stranded DNA (dsDNA) forming a right-handed helical structure (B-DNA) stimulate
cells to produce type I interferons (IFNs). While an adaptor molecule, IFN-␤ promoter stimulator 1 (IPS-1),
mediates dsDNA-induced cellular signaling in human cells, the underlying molecular mechanism is not fully
understood. Here, we demonstrate that the extrachromosomal histone H2B mediates innate antiviral immune
responses in human cells. H2B physically interacts with IPS-1 through the association with a newly identified
adaptor, CIAO (COOH-terminal importin 9-related adaptor organizing histone H2B and IPS-1), to transmit
the cellular signaling for dsDNA but not immunostimulatory RNA. Extrachromosomal histone H2B was
biologically crucial for cell-autonomous responses to protect against multiplication of DNA viruses but not an
RNA virus. Thus, the present findings provide evidence indicating that the extrachromosomal histone H2B isengaged in the signaling pathway initiated by dsDNA to trigger antiviral innate immune responses.
in hyperproduction of IFN-␤, dysregulation of erythropoiesis,
and symptoms resembling rheumatoid arthritis (12, 28). The
loss-of-function mutation of the DNase I gene has been foundin patients with systemic lupus erythematosus (SLE) and, infact, DNase I⫺/⫺ mice manifest SLE-like symptoms with antiDNA antibody (Ab) production (18, 27).
The immunostimulatory property of dsDNA is quite similar
to that of immunostimulatory RNA (isRNA), such as dsRNAand 5⬘-triphosphate RNA (2, 6). Indeed, the signaling pathways engaged by dsDNA in part are shared with those for
isRNA. Retinoic acid-inducible gene I (RIG-I) and melanoma
differentiation-associated gene 5 (MDA5) directly associatewith isRNA and trigger signaling, while it has been demonstrated that RIG-I does not directly interact with dsDNA but
mediates its signaling in a human hepatoma cell line, Huh7 (2).
IFN-␤ promoter stimulator 1 (IPS-1, also known as mitochondrial antiviral signaling), mediates the downstream signaling
induced by dsDNA or isRNA in humans, while IPS-1 solely
mediates isRNA but not dsDNA signaling in mice (2, 6, 15,22). In contrast, TANK-binding kinase 1 (TBK1) and inducibleI␬B kinase (IKKi) are essential for dsDNA- or isRNA-induced
type I IFN production in both humans and mice (2, 6). While
examining distinct molecules involved in dsDNA-mediated but
not isRNA-mediated upstream signaling, Z-DNA binding protein 1 (ZBP1, also known as DNA-dependent activator of IFN
regulatory factors [DAI]) was identified as a candidate cytosolic DNA sensor, at least in a mouse connective tissue cell line,
L929, although its in vivo role was dispensable (7, 24, 26).
Recently, a PYHIN family member, Absent in melanoma 2

Fragments of nucleic acids derived from either infectious
agents or host cells activate cell-autonomous responses to inhibit multiplication of certain viruses by inducing type I interferon (IFN) production (5). Such effects are more evident
when double-stranded DNA (dsDNA) is transduced into theintracellular compartment by use of a transfection agent orelectroporation method, suggesting that the DNA sensing system recognizes aberrant DNA fragments inside the cell (6, 21,
23). dsDNA forming a right-handed helical structure, i.e., BDNA, has a greater ability to induce type I IFNs than Z-DNA,
which has a left-handed zig-zag structure (6). dsDNA activates
type I IFN production in a wide variety of cell types, including
immune cells, such as dendritic cells and macrophages, andnonimmune cells, such as fibroblasts, epithelial cells, and thyroid cells (6, 23). Such effects of dsDNA were corroborated by
the observation in mice deficient for DNase II, in which intracellular accumulation of undegraded DNA fragments resulted* Corresponding author. Mailing address for Fumihiko Takeshita:
Department of Molecular Biodefense Research, Yokohama City University Graduate School of Medicine, 3-9 Fukuura, Kanazawaku, Yokohama
236-0004, Japan. Phone: 81-45-787-2602. Fax: 81-45-787-2851.
E-mail: takesita@yokohama-cu.ac.jp. Mailing address for Koichi Suzuki:
Department of Bioregulation, Leprosy Research Center, National Institute of Infectious Diseases, 4-2-1 Aoba-cho, Higashimurayama-shi,
Tokyo 189-0002, Japan. Phone: 81-42-391-8211. Fax: 81-42-394-9092.
E-mail: koichis@nih.go.jp.
† Supplemental material for this article may be found at http://jvi.asm.org/.
‡ K.K., F.T., and K.S. contributed equally to this work.
䌤Published ahead of print on 11 November 2009.
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Department of Molecular Biodefense Research, Yokohama City University Graduate School of Medicine, Yokohama 236-0004,
Japan1; Laboratory for Cell Function Dynamics, Advanced Technology Development Group, Brain Science Institute, RIKEN,
Wako 351-0198, Japan2; Laboratory of Host Defense, WPI Immunology Frontier Research Center, Osaka University,
Suita 565-8701, Japan3; Department of Molecular Protozoology, Research Institute for Microbial Diseases,
Osaka University, Suita 565-8701, Japan4; International Graduate School of Arts and Sciences,
Yokohama City University, Yokohama 230-0045, Japan5; and Department of Bioregulation,
National Institute of Infectious Diseases,
Higashimurayama-shi 189-0002, Japan6

VOL. 84, 2010

HISTONE H2B IN INNATE IMMUNE RESPONSES RAISED BY dsDNA(AIM2) protein, was shown to associate with an inflammasome
signaling adaptor, apoptosis-associated speck-like protein containing a CARD (ASC), and to play a critical role for caspase1 activation and interleukin-1␤ (IL-1␤) secretion in responseto dsDNA (1, 3, 4, 20).
In the present study, we show that extrachromosomal histone H2B is responsible for the dsDNA-induced type I IFN
production in human cells and for the innate immune response
to DNA virus infection.
MATERIALS AND METHODSUGG CGU C-3⬘; ZBP1B antisense, 5⬘-GAC GCC ACC AUC GGC AAC AGC
AAC A-3⬘; ZBP1C sense, 5⬘-UUC AUC CAC AUA GUG GCU GCC UUC
U-3⬘; ZBP1C antisense, 5⬘-AGA AGG CAG CCA CUA UGU GGA UGA A-3⬘;
RIG-IA sense, 5⬘-UUA GGA UUC UCA UUG CUG GGA UCC C-3⬘; RIG-IA
antisense, 5⬘-GGG AUC CCA GCA AUG AGA AUC CUA A-3⬘; RIG-IB
sense, 5⬘-AUG UCU UGU ACU UCA CAU GGA UUC C-3⬘; RIG-IB antisense, 5⬘-GGA AUC CAU GUG AAG UAC AAG ACA U-3⬘; RIG-IC sense,
5⬘-UGG ACA UGA AUU CUC ACU AAG AUU C-3⬘; RIG-IC antisense,
5⬘-GAA UCU UAG UGA GAA UUC AUG UCC A-3⬘. The cells (6 ⫻ 105)
were transfected with 120 pmol of each dsRNA by using the Lipofectamine
RNAi MAX reagent (Invitrogen) according to the manufacturer’s protocol.
Generation of mammalian expression plasmids. H2B, RIG-I, ZBP1, TBK1,
and CIAO cDNA was amplified by PCR using a human or mouse spleen cDNA
library or the isolated library plasmid as a template. cDNA fragments wereverified by sequencing and then introduced into pFLAG-CMV4 (Sigma),
pFLAG-CMV5 (Sigma), or pCIneo-HA (25), or pCAGGS-CFP, pCAGGS-YFP,
or pcDNA3-mRFP (10). To obtain full-length and truncated mutants of CIAO,
the full-length CIAO open reading frame (ORF; amino acids [aa] 1 to 249), aa
1 to 121, aa 91 to 249, or aa 194 to 249 was fused to the green fluorescent protein(GFP) ORF and introduced into pFLAG-CMV4 (FLAG-CIAO FL-GFP,
FLAG-CIAO N⬘-3a-GFP, FLAG-CIAO C⬘-4a-GFP, or FLAG-CIAO C⬘-2aGFP, respectively). To obtain a truncated mutant of histone H2B, the full-length
histone H2B ORF (aa 1 to 126), aa 1 to 103, aa 1 to 86, aa 1 to 53, or aa 1 to 37(nuclear localization signal), or aa 38 to 126 (␣-helical region) was fused to the
GFP ORF and introduced into pCIneo-HA [HA-H2B FL (N⬘-1␣2␣3␣4␣)-GFP,
HA-H2B N⬘-1␣2␣3␣-GFP, HA-H2B N⬘-1␣2␣-GFP, HA-H2B N⬘-1␣-GFP, HAH2B N⬘-tail-GFP, or HA-H2B ␣H-GFP, respectively]. To obtain full-length andtruncated mutants of IPS-1, the full-length IPS-1 ORF (aa 1 to 540), aa 1 to 100,
aa 91 to 180, aa 1 to 170, aa 1 to 100 plus aa 170 to 540, or aa 1 to 514 was either
left alone or fused to the GFP ORF and then introduced into pCIneo-HA(HA-IPS-1 FL, HA-IPS-1 CARD-GFP, HA-IPS-1 PRD-GFP, HA-IPS-1CARD-PRD, HA-IPS-1⌬PRD, or HA-IPS-1⌬TMD, respectively).
Yeast two-hybrid screening. Yeast two-hybrid screening was performed as
described previously (25). Briefly, H2B cDNA was introduced in frame into the
GAL4 DNA-binding domain of pGBKT7 as a bait plasmid (BD Clontech, Palo
Alto, CA). A Saccharomyces cerevisiae strain, AH109, was transformed with abait plasmid and the human bone marrow Matchmaker cDNA library (BDClontech). After screening 1 ⫻ 106 clones on synthetic dropout selection agar
plates (SD/-Leu/-Trp/-Ade/-His/5-bromo-4-chloro-3-indolyl-␣-D-galactopyranoside; BD Clontech), positive clones were picked and the pACT2 library plasmids
were recovered. The insert cDNA was sequenced and then characterized with
the BLAST program.
Transient transfection and reporter gene assay. The cells (1 ⫻ 104) weretransfected with 25 ng of pTK-RL (Promega) plus 25 ng of either reporter
plasmid encoding an IFN-␣4 or IFN-␤ promoter firefly luciferase (FFL) gene
cassette (pGL3 IFN-␣4 or pGL3 IFN-␤) or pNF-␬B-Luc (Stratagene). In somecases, the cells were cotransfected with expression plasmids for H2B, ZBP1, and
RIG-I. Twenty-four hours after transfection, cells were stimulated with 0.1, 0.5,
or 2.5 ␮g/ml of dsDNA [poly(dA 䡠 dT) or poly(dG 䡠 dC)] for 24 h or with 40 ng/ml
of TNF-␣ for 8 h. The Dual-Glo luciferase assay system was used to measureboth FFL activity and Renilla luciferase activity in the same sample. FFL activityof each sample was normalized against Renilla luciferase activity to obtain relative luciferase activity.
Pull-down assay. HEK293 cells were transiently transfected with H2B-FLAGand stimulated with 0.5 ␮g/ml of biotin-poly(dA 䡠 dT) or biotin-poly(dG 䡠 dC) for
4 or 16 h. Then, the cells were lysed in lysis buffer (25 mM Tris-HCl [pH 7.5], 1
mM EDTA, 0.1 mM EGTA, 5 mM MgCl2, 100 mM NaCl, 10% glycerol, and 1%
Nonidet P-40) containing both proteinase and phosphatase inhibitor cocktails(Sigma) on ice for 30 min. After centrifugation, the supernatants were recovered,
mixed with streptavidin-agarose, and rotated at 4°C for 30 min. The complexes
were washed with lysis buffer five times and suspended in SDS sample buffer, andthen the supernatants were subjected to immunoblot analysis using anti-FLAG
M2 Ab.
Viral genome binding assay. HEK293 cells were transiently transfected withhemagglutinin (HA)-GFP, HA-H2B FL-GFP, HA-H2B N⬘-tail-GFP, or HAH2B ␣H-GFP. Twenty-four hours after the first transfection, the cells weretransfected with pHPV18. Forty-eight hours after the second transfection, thecells were lysed in lysis buffer containing both proteinase and phosphatase inhibitor cocktails (Sigma) on ice for 30 min. After centrifugation, the supernatants
were recovered and HA-fusion molecules were precipitated using anti-HA Ab.
The complexes were washed with lysis buffer five times and then subjected tostandard PCR targeting the subtype-specific E6 gene using the following primer
set: HPV18 E6, 5⬘-CCT GCG GTG CCA GAA ACC GT-3⬘ and 5⬘-CGT TGG

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Cells and reagents. HEK293T, HEK293, HeLa, NIH 3T3, Vero, and L929
cells were purchased from the American Type Culture Collection (Manassas,
VA) and maintained in Dulbecco’s modified Eagle’s medium supplemented with
10% fetal calf serum and 50 ␮g/ml penicillin-streptomycin. SCC-4 squamous
carcinoma cells (JCRB9118) were obtained from Human Science Research Resources Bank (Osaka, Japan). Human umbilical vein endothelial cells (HUVEC)
were obtained from Cambrex BioScience (Waikersville, MD). IPS-1⫺/⫺ murine
embryonic fibroblasts (MEF) were previously described (15). Encephalomyocarditis virus (EMCV) and mouse cytomegalovirus (MCMV) strain MW97.01 werekindly provided by T. Fujita (Kyoto University, Kyoto, Japan) and U. H. Koszinowski (Max von Pettenkofer Institute for Virology, Munich, Germany),
respectively. poly(dA䡠dT)䡠poly(dA䡠dT) [designated poly(dA䡠dT)] and poly(dG䡠dC)䡠poly(dG 䡠 dC) [designated poly(dC 䡠 dG)] were purchased from Amersham
Biosciences (Piscataway, NJ). Poly(I:C) and human tumor necrosis factor alpha(TNF-␣) were obtained from Invivogen (San Diego, CA) and PeproTech EC(London, United Kingdom), respectively. MVAdE3L was a kind gift fromHolger Ludwig (Paul-Ehrlich Institute, Langen, Germany). The human papillomavirus (HPV) genome cloned in pBR322 (pHPV11) was obtained from JCRBGenBank (Osaka, Japan). Each nucleic acid was mixed for 15 min with TransITLT1 (Mirus Bio, Madison, WI) or Lipofectamine 2000 reagent (Invitrogen) at aconstant weight/volume ratio of 1:3 in serum-free medium before use forstimulation.
Screening of the cDNA expression library. A cDNA expression library originated from human bone marrow (Invitrogen) was screened. Escherichia coli
DH5␣ cells transformed with a human bone marrow cDNA library (Invitrogen)were subdivided into ⬃20 independent clones or 4,800 pools. Each pool was
cultured at 30°C for 48 h, and then library plasmid DNA was extracted using the
Wizard SV 96 plasmid DNA purification system (Promega, Madison, WI).
HEK293T cells stably transfected with pGL3 IFN-␤ were transiently transfectedwith each pool of the library plasmids using FuGENE6 transfection reagent.
Twenty-four hours after transfection, cells were washed and then stimulated with
0.5 ␮g/ml of poly(dA 䡠 dT). Luciferase assays were performed using the BrightGlo luciferase assay system 24 h after stimulation. The three pools that showed
the highest luciferase activities were transformed into DH5␣, and then libraryplasmid DNA from 50 independent clones was recovered. A second screening
was performed using these single plasmids in the same manner. Finally, insert
cDNA from selected clones was sequenced using a Genetic Analyzer 310 apparatus (PE Applied Biosystems, Foster City, CA) and then characterized with the
BLAST program.
RNA interference. Interference of histone mRNA was performed as described
previously (6). dsRNA was chemically synthesized by Invitrogen (stealth RNAi;
Carlsbad, CA) or iGENE (Hokkaido, Japan). Sequences of each RNA were as
follows: histone H1 sense, 5⬘-GCC CAA GAA AGU AGC UAA AAG CCC
UAG-3⬘; histone H1 antisense, 5⬘-AGG GCU UUU AGC UAC UUU CUU
GGG CAU-3⬘; histone H2A sense, 5⬘-CGC AAC GAC GAG GAA CUG AAC
AAG CAG-3⬘; histone H2A antisense, 5⬘-GCU UGU UCA GUU CCU CGUCGU UGC GAU-3⬘; histone H2Bf116 sense, 5⬘-CCG UUU ACG UGU ACAAGG UGC UGA A-3⬘; histone H2Bf116 antisense, 5⬘-UUC AGC ACC UUGUAC ACG UAA ACG G-3⬘; histone H2Bf169 sense, 5⬘-UCC AAG GCC AUG
GGC AUC AUG AAC U-3⬘; histone H2Bf169 antisense, 5⬘-AGU UCA UGAUGC CCA UGG CCU UGG A-3⬘; histone H3 sense, 5⬘-GAG AUC GCU CAGGAC UUU AAG ACC GAG-3⬘; histone H3 antisense, 5⬘-CGG UCU UAAAGU CCU GAG CGA UCU CAU-3⬘; histone H4 sense, 5⬘-GGG ACA AUA
UCC AAG GCA UUA CAA AAG-3⬘; histone H4 antisense, 5⬘-UUU GUAAUG CCU UGG AUA UUG UCC CAU-3⬘; CIAO sense, 5⬘-AUG GAC AGU
AUG AAG GCA AAG UCA GAG-3⬘; CIAO antisense, 5⬘-CUG ACU UUG
CCU UCA UAC UGU CCA UAU-3⬘; ZBP1A sense, 5⬘-AUU UCA UGU GGAUUC UCU GGG CGG C-3⬘; ZBP1A antisense, 5⬘-GCC GCC CAG AGA AUC
CAC AUG AAA U-3⬘; ZBP1B sense, 5⬘-UGU UGC UGU UGC CGA UGG

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KOBIYAMA ET AL.

RESULTS
Identification of histone H2B as a mediator of dsDNAinduced IFN-␤ promoter activation. To identify molecules
responsible for dsDNA-mediated type I IFN production in human cells, we screened a human bone marrow cDNA expression
library using HEK293T cells stably expressing the Luc gene under
the control of the IFN-␤ promoter. Among ⬎960,000 independent clones examined, one of the positive clones encompassed
the histone H2B1B ORF and exhibited a striking enhancementof dsDNA-induced IFN-␤ promoter activation (Fig. 1A). To
confirm the role of histone H2B in dsDNA-induced innate

immune responses, RNA interference was performed by usinga small interfering RNA (siRNA). H2B siRNA treatment significantly suppressed the level of H2B protein expression (seeFig. S1 in the supplemental material). H2B siRNA also suppressed the levels of IFN-␤ production and IFN regulatory
factor 3 (IRF3) phosphorylation induced by dsDNA in
HEK293 cells (Fig. 1B and C, respectively), and knockdown ofhistone H2B inhibited dsDNA-induced activation of various
gene promoters regulating IFN-related gene expression (seeFig. S2 in the supplemental material). dsDNA stimulates IRF
kinases, such as TBK1 and IKKi (6). Overexpression of TBK1
and IKKi elicited comparable levels of IFN-␤ production inboth control and H2B knockdown HEK293 cells, suggestingthat H2B functions upstream of these kinases (Fig. 1D). To
confirm the intracellular interaction of histone H2B anddsDNA, we performed pull-down and immunoprecipitation
analyses. As a result, histone H2B was coprecipitated withbiotinylated poly(dA 䡠 dT) or poly(dG 䡠 dC) that had beentransfected into cells (Fig. 1E). To further identify the dsDNA
interaction domain of histone H2B, we performed immunoprecipitation analysis using H2B mutants. It was found that the
COOH-terminal ␣-helical region (␣H domain) of histone H2B
is sufficient for the interaction with HPV DNA (Fig. 1F). Furthermore, complementation of full-length histone H2B inknockdown cells restored the levels of IFN-␤ promoter activation induced by dsDNA but not that of the NH2-terminal tailregion (N⬘-tail) or the ␣H domain alone (Fig. 1G). These
results suggest that histone H2B binds to dsDNA through the␣H domain, but both the N⬘-tail and ␣H domain are requiredfor dsDNA-mediated signaling. When different siRNAs, eachtargeting histone H1, H2A, H2B, H3, or H4 were tested, only
H2B siRNA attenuated IFN-␤ promoter activation induced by
dsDNA (Fig. 1H). Such an effect of H2B siRNA was notobserved following dsRNA or TNF-␣ stimulation (Fig. 1H).
These results suggest that histone H2B is involved in the signal
activation triggered by dsDNA.
Histone H2B is involved in the signal activation triggered by
dsDNA but not that by dsRNA nor TLR ligands. ZBP1 or
RIG-I is known as a cytosolic DNA sensor in a mouse connective tissue cell line, L929, and a human hepatoma cell line,
Huh7, respectively. dsDNA-induced IFN-␤ promoter activation was significantly suppressed in the cells treated with histone H2B siRNA, but not with siRNA targeting ZBP1 or
RIG-I (Fig. 2A), although each siRNA specifically downregulated the level of target protein expression in HEK293 cells(Fig. 2B). Knockdown of histone H2B resulted in a suppression of IFN-␤ production in response to dsDNA, but not to5⬘-triphosphate RNA stimulation in HUVEC (Fig. 2C). Alt
hough histone H2B is an essential component of thechromosome, H2B knockdown cells used in our present study
could normally proliferate and respond to TNF-␣ or Toll-likereceptor (TLR) ligands comparably to those treated with con
trol siRNA (Fig. 2D and E), suggesting that chromosomal
histone H2B that is essential for cell survival or TNF receptor(TNFR)- or TLR-mediated signaling pathways were not impa
ired by the use of these siRNAs. These results in toto suggestthat histone H2B is involved in the signal activation triggeredby dsDNA, but not that by dsRNA or TLR ligands.
Extrachromosomal histone H2B associates with IPS-1 in thecytoplasm. Immunofluorescence analysis revealed that al-

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AGT CGT TCC TGT CG-3⬘. The PCR products were separated on 2% agarosegels and visualized under UV light after ethidium bromide staining.
Immunoprecipitation and immunoblotting analysis. Immunoprecipitationand immunoblotting analyses were performed using anti-ZBP1 (clone RG7D12;
kindly provided by Stefan Rothenburg [National Institutes of Health]), anti-H2B(BioVision, Mountain View, CA), anti-phospho-IRF-3 (Ser396) 4G4D, antiIRF3 (Cell Signaling, Danvers, MA), anti-RIG-I (AnaSpec, Inc., San Jose, CA),anti-IPS-1 (MAVS) (Bethyl, Montgomery, TX), anti-importin 9, anti-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) 6C5 (Abcam, Cambridge, United
Kingdom), anti-Sp1, anti-STAT1, anti-extracellular signal-regulated kinase(ERK), anti-phospho-STAT1 (Santa Cruz Biotechnology, Santa Cruz, CA), antiFLAG M2 (Sigma), or anti-HA Ab (Roche Diagnostics, Indianapolis, IN) as
described previously (25).
FRET. For flow cytometric analysis of fluorescence resonance energy transfer(FRET), 293-F cells (Invitrogen) were transfected with cyan fluorescent protein(CFP) and/or yellow fluorescent protein (YFP) fusion proteins encoding histone
H2B, IPS-1, or CIAO in 293 expression medium (Invitrogen) for 24 h, andmeasurements of YFP (excitation, 488 nm; emission, 530 nm), CFP (excitation,
407 nm; emission, 510 nm), FRET (excitation, 407 nm; emission, 535 nm) were
performed using FACSAria (Becton Dickinson) and BD FACSDiVa software.
FRET is shown as the YFP emission obtained by CFP excitation divided by the
CFP emission by CFP excitation (11).
Transient replication of HPV. SCC-4 cells treated with control siRNA or H2B
siRNA were transfected with the pHPV11 or -18 (VG012 and VG013; Human
Science Research Resources Bank, Osaka, Japan) using Lipofectamine 2000
transfection reagent (Invitrogen). After incubation at 37°C for 24, 36, 60, or 72 h,
the cells were washed with phosphate-buffered saline (PBS) and suspended in
Hirt lysis buffer (0.6% SDS and 20 mM EDTA, pH 8.0). After 15 min, NaCl wasadded to a final concentration of 1 M, and then the mixture was placed at 4°C for
12 h. After centrifugation, the supernatants were treated with phenol-chloroform, and then DNA was precipitated with isopropanol. To distinguish replicatedDNA from input DNA (pHPV11 or pHPV18), the samples were treated with
DpnI endonuclease to degrade input DNA that had been propagated in E. coli
DH5␣. Resultant samples were suspended in Tris-EDTA, the concentrations
were adjusted by measuring the optical density at 260 nm, and then 200 ng of
DNA sample was subjected to standard PCR, targeting the subtype-specific E6gene using the following primer sets: HPV11 E6, 5⬘-CTC CAC GTC TGC AAC
ATC TA-3⬘ and 5⬘-TGA CAC AGG TAA CAA CGA AT-3⬘; HPV16 E6,5⬘-CAC CAA AAG AGA ACT GCA ATG-3⬘ and 5⬘-TCA CGT CGC AGTAAC TGT TG-3⬘. The PCR products were separated on 2% agarose gels andvisualized under UV light after ethidium bromide staining.
Virus multiplication study. Twenty-four hours after HEK293 or NIH 3T3 cells
were infected with adenovirus (AdV) type 5 or MCMV strain MW97.01, respectively, total DNA was collected using a DNeasy tissue kit (Qiagen). One hundred
nanograms of each sample was subjected to PCR using primers for the AdV
hexon 5 gene, 5⬘-TGA AGC TGC TAC TGC TCT TGA A-3⬘ and 5⬘-GCA TTC
AAC TGC CAT GCT TGG C-3⬘ (18 cycles), the human IFN-␤ gene promoter
region (⫺110 to ⫹20), 5⬘-CTA AAA TGT AAA TGA CAT AGG-3⬘ and5⬘-AAA GGT TGC AGT TAG AAT GTC-3⬘ (18 cycles), the MCMV DNA
polymerase gene, 5⬘-ACG CCG AGA AAG AGT ACG TGC TCA A-3⬘ and5⬘-TCG GAC TGC ATC CTC TCG CAG TAG-3⬘ (22 cycles), or the mouse
Cd63 gene, 5⬘-GGT CTT GGG AAT TAT CTT CTC CTG CTG-3⬘ and 5⬘-CAC
AGG CGG CAA AAT TCT TAA ACA TTC-3⬘ (26 cycles). After cell supernatants were recovered, the number of infectious viruses in 1 ml of each sample wasdetermined by viral plaque assay or the 50% tissue culture infective dose
(TCID50) as described previously (10).
Statistical analysis. Student’s t test was used for statistical analyses.

J. VIROL.

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825

though most histone H2B colocalized with Hoechst staining(chromosomes), there was some H2B staining in the extrachromosomal area (Fig. 3A). The levels of such extrachromosomal H2B were diminished when cells were treated with H2BsiRNA, while chromosomal H2B in the same cell was present

at a level similar to that in the control cells. Since it is known
that histones are also released from the nucleus (14), the levelsof histone H2B in each cellular fraction were evaluated byimmunoblotting analysis. In accordance with the results ofimmunofluorescence analysis, H2B siRNA significantly de-

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FIG. 1. Histone H2B mediates dsDNA-induced cellular activation. (A) A human bone marrow expression cDNA library was screened basedon the ability to mediate dsDNA-induced IFN-␤ promoter activation. (B to D) HEK293 cells were transfected with control siRNA or siRNAtargeting H2B. (B) The cells were treated with 0, 0.1, 0.5, or 2.5 ␮g/ml of dsDNA [poly(dA 䡠 dT)], and the supernatants were subjected to an
enzyme-linked immunosorbent assay (ELISA) for IFN-␤ (PBL). (C) The cells were treated with 0.5 ␮g/ml of dsDNA [poly(dA 䡠 dT)] for 0, 3, 6,
12, 24, and 48 h, and the levels of phosphorylated IRF3 (p-IRF3) in the nucleus and IRF3 (normalization control) in the whole-cell lysates were
examined by immunoblotting analysis. (D) The cells were further transfected with expression plasmids for TBK1 and IKKi, and the supernatants
were subjected to ELISA for IFN-␤ (PBL). (E) HEK293 cells were transfected with histone H2B-FLAG and treated with biotin-poly(dA 䡠 dT) or
biotin-poly(dG 䡠 dC). The cell lysates were collected, and a pull-down assay was performed with streptavidin-agarose. The complex was analyzed
by immunoblotting using anti-FLAG Ab. (F) HEK293 cells were transfected with HA-GFP, HA-H2B-GFP, HA-H2B N⬘-tail-GFP, or HA-H2B
␣H-GFP. Twenty-four hours after the first transfection, the cells were further transfected with the HPV18 genome. Forty-eight hours after thesecond transfection, the cell lysates were collected and immunoprecipitation was performed with anti-HA Ab. The standard PCR targeting the
HPV18 E6 gene was conducted with each cell lysate and immunoprecipitated complex. (G) HEK293 cells were transfected with H2B siRNA andthen further transfected with pGL3 IFN-␤ and pTK-RL plus control, H2B FL, the H2B N⬘-tail, or the H2B ␣H plasmid. After treatment with 0,0.1, 0.5, or 2.5 ␮g/ml of dsDNA [poly(dA 䡠 dT)] for 24 h, a luciferase assay was performed. (H) HEK293 cells were transfected with control siRNAor siRNA targeting either H2B, H1, H2A, H3, or H4. The cells were further transfected with pGL3 IFN-␤ and pTK-RL and treated with or without
0.5 ␮g/ml of dsDNA [poly(dA 䡠 dT)], dsRNA [poly(I:C)], or TNF-␣ (50 ng/ml) for 24 h, and then a luciferase assay was performed. The luciferase
activity is depicted as the IFN-␤ promoter activity relative to samples obtained from the cells treated with control siRNA in each stimulation(relative IFN-␤ promoter activity). All data except for those in panel represent means ⫾ standard deviations (SD) of six to eight samples. *, P ⬍
0.05.

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creased extrachromosomal H2B, but not choromosomal H2B(Fig. 3B). Only when the cells were stimulated with dsDNA for
12 h, but not when they were stimulated with dsRNA, was apunctate signal of H2B fluorescence detected in the cytoplasm(Fig. 3C), suggesting that dsDNA induces aggregation of histone H2B in the cytoplasm. Of interest, the colocalization of
H2B with IPS-1 was observed following dsDNA stimulation(Fig. 3C, magenta arrows). To further examine the molecularinteraction between histone H2B and IPS-1, immunoprecipitation analysis was performed. The cytoplasmic extracts andpostchromosomal nuclear extracts were segregated from
HEK293 cells stimulated with or without dsDNA. The purityof each extract was confirmed by immunoblotting analysis with
anti-GAPDH (a cytoplasmic enzyme) or anti-Sp1 (a nuclear
transcription factor) Ab as shown in Fig. 3D. IPS-1 was predominantly present in the cytoplasmic extracts, while histone
H2B was in both extracts. When IPS-1 was immunoprecipitated from the cytoplasmic extracts, coprecipitation of histone
H2B was not detected before stimulation but was detected 12 hafter dsDNA stimulation. In contrast, when H2B was immunoprecipitated from the nuclear extracts, no detectable IPS-1

coprecipitated even after dsDNA stimulation. Taken together,
these results suggest that the interaction between histone H2B
and IPS-1 takes place in the cytoplasm following dsDNA stimulation.
Identification of CIAO as a species-specific molecule thatlinks histone H2B and IPS-1. To identify the molecules directly associating with histone H2B, yeast two-hybrid screening
was performed. As a result, 3 out of 10 positive clones wereidentified as KIAA1192, whose function has not yet beencharacterized but this nucleotide sequence is identical to the
COOH-terminal end of importin 9, as a possible alternative
splicing transcript of importin 9 (see Table S1 in the supplemental material). We renamed it to CIAO, based on its novelrole, described below. Immunoprecipitation analysis clearly
revealed that CIAO binds to histone H2B as well as IPS-1 but
not to RIG-I (Fig. 4A). Full-length importin 9 interacted withhistone H2B but not with IPS-1 (data not shown), suggestingthat CIAO, a possible alternative splicing variant of importin 9,
gains a peculiar function as an interacting partner of IPS-1.
The spatial disposition of H2B, CIAO, and IPS-1 in the cell
was further examined by FRET analysis. As shown in Fig. 4B,

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FIG. 2. Histone H2B is involved in the signal activation triggered by dsDNA, but not that of dsRNA or TLR ligands. HEK293 (A, B, D, andE) or HUVEC (C) cells were transfected with control siRNA or siRNA targeting either H2B, ZBP1, or RIG-I. (A and B) The cells were furthertransfected with expression plasmids for histone H2B, ZBP1, and RIG-I, in the presence of pGL3 IFN-␤ plus pTK-RL. (A) Twenty-four hours
after transfection, cells were treated with 0, 0.1, 0.5, or 2.5 ␮g/ml of dsDNA [poly(dA 䡠 dT)] for 24 h, and then a luciferase assay was performed.
(B) Twenty-four hours after transfection, levels of protein expression were examined by immunoblotting analysis using anti-RIG-I, anti-ZBP1,
anti-H2B, or anti-␤-actin Ab. (C) The cells were treated with 0.1 or 1.0 ␮g/ml of dsDNA [poly(dA 䡠 dT)] or 5⬘-triphosphate RNA (ppp-RNA) for
24 h, and the supernatants were subjected to an ELISA for IFN-␤. (D) The cells were further transfected with pNF-␬B-Luc plus pTK-RL andtreated with or without TNF-␣ (50 ng/ml) for 8 h, and then a luciferase assay was performed. (E) The cells were further transfected with
pNF-␬B-Luc plus pTK-RL in the presence of either expression plasmid for TLR2 or TLR4 plus MD2 and treated with or without peptidoglycan(PGN; 1 ␮g/ml) or lipopolysaccharide (LPS; 100 ng/ml) for 24 h, and then a luciferase assay was performed. *, P ⬍ 0.05; ***, P ⬍ 0.001.

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FIG. 3. Extrachromosomal histone H2B interacts with IPS-1. (A) HeLa cells were transfected with control or H2B siRNA. Twelve hours after
transfection, the cells were fixed, stained with Hoechst 33258 and anti-H2B Ab followed by Alexa 488-conjugated anti-mouse IgG Ab, and thenexamined under a confocal microscope. The pictures taken under low and high lighting conditions are shown as low and high, respectively. Bar,
20 ␮m. (B) The cells were collected 0, 6, or 12 h after H2B siRNA treatment, and the cell lysates were fractionated into the cytoplasmic extract,
the nuclear extract, and the chromosome fraction. Each fraction was analyzed by immunoblotting using anti-H2B, anti-GAPDH, or anti-Sp1 Ab.
The density of each H2B band was normalized to the density of the corresponding GAPDH band (the cytoplasmic extract) or Sp1 band (the nuclear
extract and the chromosome fraction) and is shown on the graph (n ⫽ 3). (C) After stimulation with or without dsDNA or dsRNA for 12 or 24 h,
HeLa cells were fixed and stained with anti-H2B Ab and anti-IPS-1 Ab followed by Alexa 488-conjugated anti-mouse IgG Ab and Alexa
555-conjugated anti-rabbit IgG Ab. The cells were then examined under a confocal microscope. Bar, 100 ␮m in low-magnification pictures (upper
panels) and 50 ␮m in high-magnification pictures (lower panels). (D) HEK293 cells were collected 0, 12, or 24 h after dsDNA stimulation, andthe cell lysates were fractionated into the cytoplasmic extract and nuclear extract. Each fraction was immunoprecipitated with control IgG,
anti-H2B, or anti-IPS-1 Ab. The immune complexes were analyzed by immunobloting using either anti-H2B or anti-IPS-1 Ab. *, P ⬍ 0.05.

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FIG. 4. CIAO associates with histone H2B and IPS-1 to constitute a signaling complex, thereby transmitting a signal leading to type I IFN
production. (A and C to F) Cell lysates from HEK293 cells transfected with the indicated expression plasmid were immunoprecipitated with
anti-FLAG Ab. The immune complexes were analyzed by immunoblotting using either anti-HA or anti-FLAG Ab. (B) 293-F cells transfected with
CFP and YFP fusion proteins encoding histone H2B, IPS-1, or CIAO. FRET results are shown as the YFP emission obtained by CFP excitation
divided by the CFP emission by CFP excitation. (C to F) Schematic diagrams of CIAO, IPS-1, H2B, and their truncated mutants. ␣, a putative
␣-helical region of CIAO and H2B; *, a predicted gene product of the target molecule. (G) HeLa cells were transfected with control siRNA or
CIAO siRNA. Immunoblotting analysis was conducted using anti-importin 9 (COOH-terminal end) monoclonal Ab. (H and I) HeLa cells weretransfected with or without the indicated reporter plasmid plus the indicated amounts (in ng) of expression plasmid(s). A luciferase assay was
performed as described for Fig. 1. Data represent means ⫾ SD of the relative luciferase activity of six to eight samples. *, P ⬍ 0.05.

the higher levels of the specific FRET signal were detected incells expressing CFP-CIAO and YFP-H2B as well as CFPCIAO and YFP-IPS-1, compared to those expressing CFPH2B and YFP-IPS-1, suggesting that CIAO and H2B or CIAO
and IPS-1 juxtapose in a close spatiality and that CIAO dis-poses between H2B and IPS-1 in a molecular complex, thereby
acting as an adaptor linking histone H2B to IPS-1. We further
characterized the domains of CIAO, histone H2B, and IPS-1
that are responsible for the molecular interaction. Immunoprecipitation analysis demonstrated that the COOH-terminal

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829

region of CIAO (aa 194 to 249) is sufficient to interact with
IPS-1, while the central region of CIAO (aa 91 to 193) isrequired for interaction with H2B (Fig. 4C and D). The N⬘-tailof H2B (aa 1 to 37) is sufficient, while the proline-rich domain(PRD) of IPS-1 (aa 94 to 170) is required for the interaction
with CIAO (Fig. 4E and F). CIAO was endogenously expressed in HeLa and HEK293 cells (Fig. 4G and data notshown), and the treatment of siRNA targeting the CIAO ORF
significantly suppressed the expression level of CIAO but notimportin 9 protein (Fig. 4G). Knockdown of CIAO resulted ina marked reduction in dsDNA-induced activation of the IFN-␤
promoter (Fig. 4H), while overexpression of CIAO enhanced
IPS-1-induced activation of type I IFN promoters (Fig. 4I).
Taken together, these results indicate that CIAO links histone
H2B to IPS-1, thereby enabling H2B-mediated type I IFN
production in human cells.
Species-specific involvement of histone H2B and IPS-1 in
dsDNA-mediated signaling. Previous reports have demonstrated that there are mechanistic differences in dsDNA signaling among species, e.g., human IPS-1 but not mouse IPS-1
is involved in dsDNA-mediated cellular signaling (2, 6, 15, 22).
To examine a molecular mechanism underlying the speciesspecific signaling pathway induced by dsDNA, we compared
the effects of H2B knockdown in human and mouse cells. Our
results showed that knockdown of H2B resulted in a suppression of dsDNA-mediated IFN-␤ promoter activation in HeLa
cells but not in NIH 3T3 cells (Fig. 5A and B), suggesting that
there is a difference in the action of histone H2B between these

species. Of interest, however, although an interaction was observed between human CIAO and IPS-1, an interaction between mouse CIAO and IPS-1 was not observed (Fig. 5C).
While high similarities of amino acid sequences were seenamong human and mouse H2B subtypes (human H2B1A, -1B,
-1C, -1D, -1H, -1J, -1K, -1L, -1M, -1N, -1O, -2E, -2F, -3B, and-FS and mouse H2B1A, -1B, -1C, -1F, -1H, -1K, -1M, -1P,
and -3A) (⬎70.1%) and between human and mouse CIAO
(99.2%), a lower level of identity was seen between human andmouse IPS-1 amino acid sequences (30.3%). Thus, it was suggested that human IPS-1, but not mouse IPS-1, has the potential to interact with histone H2B and transmit signaling. To
further examine a human IPS-1-specific mechanism in dsDNAmediated signaling, we complemented IPS⫺/⫺ mouse embryonic fibroblasts (MEF) with mouse IPS-1 (mouse IPS-1 MEF)
or human IPS-1 (human IPS-1 MEF) and tested their responses
to dsDNA. While Ifnb1 mRNA expression was induced within
3 h, its level peaked at 12 h after dsDNA stimulation and thereafter declined in mouse IPS-1 MEF (Fig. 5D). In contrast, thelevel of Ifnb1 mRNA expression peaked at 24 h after dsDNA
stimulation in human IPS-1 MEF, and the overall level was significantly higher in human IPS-1 MEF than in mouse IPS-1 MEF(Fig. 5D). Thus, these results, taken together, suggest that human
IPS-1 mediates the later-phase induction of IFN-␤ compared withthat mediated by the dsDNA signaling endogenously present in
MEF (Fig. 5D).
Extrachromosomal histone H2B is involved in suppressionof DNA virus multiplication. We further evaluated the biolog-

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FIG. 5. Species-specific involvement of histone H2B and IPS-1 in dsDNA-mediated signaling. (A and B) HeLa or NIH 3T3 cells weretransfected with either control or H2B siRNA. (A) The cells were further transfected with pGL3 IFN-␤ and pTK-RL and treated with 0, 0.1, 0.5,
or 2.5 ␮g/ml of dsDNA [poly(dA 䡠 dT)] for 24 h, and then a luciferase assay was performed. (B) The levels of H2B protein expression were examinedby immunoblotting analysis. (C) The immunoprecipitation analysis was performed after HEK293 cells were transfected with the expression
plasmids for human or mouse IPS-1 and CIAO. (D) IPS-1⫺/⫺ MEF stably complemented with mouse or human IPS-1 were stimulated with 0.5
␮g/ml of dsDNA [poly(dA 䡠 dT)] for 3, 6, 12, 24, 36, and 48 h. The cells were subjected to real-time PCR analysis for Ifnb1 mRNA and 18S rRNA.
The levels of Ifnb1 mRNA were normalized with the corresponding levels of 18S rRNA and are shown on the graph (n ⫽ 4). *, P ⬍ 0.05; **, P⬍ 0.01.

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ical role of histone H2B on cell-autonomous antiviral responses. Knockdown of histone H2B suppressed vaccinia virusinduced IFN-␤ production as well as STAT1 phosphorylation(Fig. 6A). Multiplication of human papilloma viruses (HPV 11
and HPV 16) and AdV type 5 was significantly enhanced in the
H2B knockdown cells (Fig. 6B and C, respectively) but not in
RIG-I or ZBP-1-knockdown cells (see Fig. S3 in the supplemental material). H2B knockdown, however, had virtually noeffect on the multiplication of MCMV in mouse cells and RNA
virus, EMCV, in human cells (Fig. 6D and E). These findings
suggest that extrachromosomal histone H2B is involved in asensing mechanism of DNA virus infection and mediates cellautonomous antiviral innate immune responses in human cells.
DISCUSSION
Our results provide direct evidence indicating that extrachromosomal histone H2B plays an important role in the signaling pathway triggered by dsDNA. Biologically, dsDNA is released following unusual or “dangerous” situations in cells,e.g., infection, apoptosis, and tissue damage (6, 18, 23). Therefore, the signaling pathway mediated by the signalsome con-

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FIG. 6. Histone H2B is a crucial element for suppression of DNA virus replication. (A to E) HEK293 (A and C), SCC-4 (B), NIH 3T3 (D),or HeLa cells (E) were pretreated with control siRNA or H2B siRNA. (A) The cells were infected with MVAdE3L (1 ⫻ 105 or 1 ⫻ 107 PFU).
Twenty-four hours after infection, cell lysates were subjected to immunoblotting for total STAT1, phosphorylated STAT1 (p-STAT1), or ERK.
(B) The cells were transfected with the HPV11 or -16 genome, and the episomal DNA fractions were recovered 24, 36, 60, or 72 h after
transfection. Viral multiplication was determined by PCR. (C and D) The cells were infected with AdV type 5 or MCMV strain MW97.01.
Twenty-four hours after infection, viral multiplication was determined by PCR amplification of the viral genomic DNA and by a plaque assay(AdV) or by measuring the TCID50 (MCMV). (E) The cells were infected with EMCV. Twenty-four hours after infection, viral multiplication wasdetermined by a plaque assay. *, P ⬍ 0.05; **, P ⬍ 0.01.

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mediated signaling in mouse L929 cells, its in vivo role isredundant (7, 24).
The present data indicate that extrachromosomal histone
H2B recognizes and interacts with not only “transfected”
dsDNA but also viral DNA exposed to the cytoplasm. Thismechanism is crucial for cell-autonomous innate responses toinfection with vaccinia virus, AdV, and HPV, but not with
EMCV. Our ongoing analysis shows that replication of HIV isalso enhanced in H2B knockdown Magic5 cells (data notshown), suggesting that proviral dsDNA rather than genomic
RNA of HIV is sensed by extrachromosomal histone H2B.
Thus, histone H2B seems to discriminate between foreign
DNA and RNA upon viral infection to evoke IPS-1-mediatedsignaling through the association with a novel adaptor protein,
CIAO, and it is suggested that human IPS-1 has evolutionarily
gained the potential to transmit dsDNA- and histone H2Bmediated signaling to combat against human viruses that produce DNA intermediates within the cell.
In conclusion, our present work demonstrates that extrachromosomal histone H2B physically interacts with IPS-1
through CIAO to form a distinct signaling complex thattransmits dsDNA-induced type I IFN production in humancells. Such a molecular platform may act as a sensor of thedsDNA aberrantly present within the cell, alerting cells tourgent hazards, such as pathogens (infection), apoptosis(hormonal stimuli, chemical agents, or irradiation), and necrosis (injury). Thus, this mechanism may also play some roles
in autoimmunity, transplantation rejection, gene-mediatedvaccines, and other therapeutic applications.
ACKNOWLEDGMENTS
We declare no conflicts of interest.
This work was supported in part by the Strategic Research Project of
Yokohama City University (K18022 to F.T.), Advancement of Medical
Sciences from Yokohama Medical Foundation (to F.T.), the National
Institute of Biomedical Innovation (to K.O.), Mitsubishi Pharma Research Foundation (to K.S.), and a Grant-in-Aid for Scientific Researches (B and C) and Innovative Areas (18590432, 20590477, and20200074 to F.T., 18659126 to K.O., and 15390296 to K.S.) from the
Ministry of Education, Culture, Sports, Science, and Technology ofJapan and from the Japan Science and Technology Corporation.
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