Interleukin-1 and Tumor Necrosis Factor-α Trigger Restriction of Hepatitis B Virus Infection via a Cytidine Deaminase Activation-induced Cytidine Deaminase (AID)

Background: Cytokines and host factors triggering innate immunity against hepatitis B virus (HBV) are not well understood. Results: IL-1 and TNFα induced cytidine deaminase AID, an anti-HBV host factor, and reduced HBV infection into hepatocytes. Conclusion: IL-1/TNFα reduced host susceptibility to HBV infection through AID up-regulation. Significance: Proinflammatory cytokines modulate HBV infection through a novel innate immune pathway involving AID.

The intracellular immune response can eliminate pathogens from a host, and host cells possess different mechanisms to counteract viral infection depending on the virus type. Human immunodeficiency virus (HIV) infection is restricted by cellular proteins designated as restriction factors, including APOBEC3G (A3G), 3 TRIM5␣, tetherin/BST-2, and SAMHD1 (1,2). All of these factors can be induced by stimulation with interferon (IFN). Hepatitis C virus (HCV) is eliminated by type I and III IFNs derived from dendritic cells or infected hepatocytes (3)(4)(5)(6). In hepatocytes, this process involves a series of antiviral factors that are downstream genes of IFN, IFN-stimulated genes (ISGs). Influenza virus spread and virulence is inhibited by cytokines such as IFNs and TNF␣. Responsive genes for these mechanisms include IFN-induced cellular Mx proteins that are dynamin-like GTPases (7,8). However, these cytokine-induced antiviral immune responses are poorly understood in hepatitis B virus (HBV) infection.
HBV infection is a worldwide health problem affecting more than 350 million people and is a major cause of the development of liver cirrhosis and hepatocellular carcinoma (9 -11). During the course of infection, a number of cytokines and chemokines are up-regulated in HBV-infected patients, including IFN␣/ ␥/, TNF␣, IL-1, IL-6, IL-10, IL-12, IL-15, and IL-8 (12)(13)(14)(15). Some of these cytokines are reported to suppress HBV replication (3, 16 -21). In particular, type I, II, and III IFNs suppress the replication of HBV in vitro and in vivo (19,20,(22)(23)(24)(25)(26). Although one of the downstream genes of IFN, A3G, has the potential to reduce HBV replication (27)(28)(29)(30)(31)(32)(33)(34), it is still under discussion whether this protein is responsible for the anti-HBV activity of type I IFN, because it has been previously reported by Trono and co-workers (28,35) that the induction of A3G does not explain the IFN-induced inhibition of HBV replication. Moreover, these studies were carried out using an HBV transgene that only reproduces a portion of the whole HBV life cycle, mainly focusing on intracellular HBV replication.
Here, we screened for cytokines and chemokines that affected HBV infection in HepaRG cells, a human hepatocyte cell line susceptible to HBV infection and reproducing the whole HBV life cycle (36,37). IL-1 and TNF␣ decreased the host cell permissiveness to HBV infection, and this effect was at least partly mediated by the induction of activation-induced cytidine deaminase (AID). The anti-HBV activity of IL-1/TNF␣ was mechanistically different from that of IFN␣. This study presents the activity of IL-1/TNF␣ to suppress HBV infection into hepatocytes independent of the effect on immune cells and the physiological role of AID in this machinery. Moreover, as far as we know, this is the first report to show the AID function to inhibit the infection of human pathogenic virus.
HBV Preparation and Infection-HBV used in this study was mainly derived from HepAD38 cells, which is classified as genotype D (38). Media from HepAD38 cells at days 7-31 postinduction of HBV by depletion of tetracycline were recovered every 3 days. Media were cleared through a 0.45-m filter and precipitated with 10% PEG8000 and 2.3% NaCl. The precipitates were washed and resuspended with medium at ϳ200-fold concentration. The HBV DNA was quantified by real time PCR. HBV genotype A and C in Fig. 7B was recovered from the media of HepG2 cells transfected with the plasmid pHBV/Aeus and pHBV/C-AT (41).
HepaRG cells were infected with HBV at 2000 (Fig. 7B) or 6000 (other figures) genome equivalent (GEq)/cell in the presence of 4% PEG8000 for 16 h as described previously (36). Urban and co-workers (42) reported that more than 10 3 GEq/ cell amount of HBV derived from HepAD38 or HepG2.2.15 cells (i.e. 1.25-40 ϫ 10 4 GEq/cell) as inoculum was required for efficient infection into HepaRG cells. The anti-HBV effect of IL-1/TNF␣ shown in this study was also observed when inoculated with HBV at 300 GEq/cell (data not shown).
Extraction of DNA and RNA-HBV DNA was extracted from the cells or from the medium using a DNA kit (Qiagen) according to the manufacturer's protocol. Total RNA was recovered with RNeasy mini kit (Qiagen) according to the manufacturer's protocol.
ELISA-HBs protein was quantified by ELISA using plates incubated at 4°C overnight with a sheep anti-HBs antibody at 1:5000 dilution (Maxisorp nunc-immuno plate, Nunc catalog no. 439454) followed by coating with 0.2% BSA, 0.02% NaN 3 , 1ϫ PBS at 4°C until use. Samples were incubated with the plates for 2 h and after washing with TBST four times, horseradish peroxidase-labeled rabbit anti-HBs antibody was added for 2 h. The substrate solution (HCV core ELISA kit: Ortho) was reacted for 30 min before the A 450 values were measured.
Indirect Immunofluorescence Analysis-Indirect immunofluorescence analysis was performed essentially as described previously (45). After fixation with 4% paraformaldehyde and permeabilization with 0.3% Triton X-100, an anti-HBc antibody (DAKO, catalog no. B0586) was used as the primary antibody.
MTT Assay-The MTT assay was performed as described previously (46).
Immunoblot Analysis-Immunoblot analysis was performed as described previously (47). The polyclonal antibody against AID was generated using a peptide derived from AID protein as an immunogen as described previously for preparation of the anti-AID antibody 1 (48). The specificity of the antibody was described previously (48,49).
Southern Blot Analysis-Southern blot was performed as described previously (41). After digestion of free nucleic acids with DNase I and RNase A, cell lysates were digested with proteinase K, and HBV DNA in the core particles was extracted with phenol/chloroform, followed by isopropyl alcohol precipitation. Probe was prepared by cutting pHBV/D-IND60 (41) with SacII and BspHI to generate a full-length HBV DNA probe and labeled with AlkPhos direct labeling reagents (GE Healthcare). Labeled bands were visualized with CDP-star detection reagent (GE Healthcare).
Quantification of Nucleocapsid-associated HBV RNA-After digestion of free nucleic acids with DNase I and RNase A, nucleocapsid was precipitated with PEG8000 (41). Total RNA was then extracted from the resuspended precipitates. HBV RNA was quantified by real time RT-PCR with 5Ј-TCC-CTCGCCTCGCAGACG-3Ј and 5Ј-GTTTCCCACCTTAT-GAGTC-3Ј as primers with Power SYBR Green PCR Master Mix (Applied Biosystems).
Differential DNA Denaturation PCR-Differential DNA denaturation PCR was performed as described previously (51).
Reporter Assay-DNA transfection was performed with pNF-B-luc or pISRE-TA-luc (Stratagene) and pRL-TK (Promega), which express firefly luciferase driven by NF-B or ISRE and Renilla luciferase by herpes simplex virus thymidine kinase promoter, respectively, and Polyethylenimine Max (Polysciences Inc., catalog no. 24765). After compound or cytokine treatment, cells were lysed, and luciferase activities were measured as described previously (52). A reporter carrying HBV core promoter was constructed by inserting the DNA fragment (1413-1788 nucleotide number) of HBV DNA (D-IND60) into pGL4.28 vector (Promega) (41). In the reporter assay using this construct (Fig. 1H), HX531, a retinoid X receptor antagonist was used as a positive control as retinoid X receptor was involved in the transcription from the core promoter (53).

IL-1 Reduced Host Cell Susceptibility to HBV Infection-To
evaluate the effect of cytokines and chemokines on susceptibility to HBV infection, we treated HepaRG cells (36) with cytokines for 3 h prior to and 16 h during HBV infection, followed by culture without stimuli for an additional 12 days (Fig. 1A, lower scheme). Heparin, a competitive inhibitor of HBV attachment (54), was used as a positive control and decreased secretion of the viral envelope surface protein (HBs) from HBV-infected cells ( (Fig. 1G). HBV cccDNA and HBV RNA was also decreased in infected cells treated with IL-1␤ (Fig. 1, E and F). IL-1␤ did not decrease HBV core promoter activity at least in HepG2 cells (Fig. 1H). These results suggest that IL-1␤ suppressed HBV infection to HepaRG cells. IL-1␤ did not decrease the expression of sodium taurocholate cotransporting polypeptide (NTCP), a recently reported HBV entry receptor (data not shown) (55). Similar results were obtained using primary human hepatocytes (Fig.  1I).
NF-B Signaling Was Critical for Anti-HBV Activity-As shown in Fig. 2A, IL-1␤ suppressed HBV infection in a dose-dependent manner. This anti-HBV effect was reversed by cotreatment with a neutralizing antibody for the IL-1 receptor, IL-1RI (Fig. 2B), suggesting that receptor engagement was required for anti-HBV activity. IL-1Ra is a natural antagonist that associates with IL-1RI but does not trigger downstream signal transduc-tion (56). Treatment with IL-1Ra did not decrease HBV infectivity (Fig. 2C), suggesting that signal transduction triggered by IL-1 was required for anti-HBV activity.
To identify the signal transduction pathway essential for anti-HBV activity, we treated HepaRG cells with PD98059, SP600125, SB203580, and Bay11-7082, which are inhibitors for MEK, JNK, p38, and NF-B, respectively (57). As shown in Fig.  2D, only cotreatment with Bay11-7082 significantly removed the anti-HBV effect of IL-1␤. Luciferase assay and RT-PCR analysis indicated that Bay11-7082, but not other inhibitors, blocked the transactivation of NF-B (Fig. 2E, upper panels) and NF-B downstream genes, cIAP and XIAP (Fig. 2E, lower panels). Additional NF-kB inhibitors, BMS-345541 and QNZ (Fig. 2G), also reversed the anti-HBV effect of IL-1␤ (Fig. 2F). These data suggest a critical role for NF-B activation in the anti-HBV activity. Additionally, IL-1␤ did not augment the activity of interferon sensitivity-responsive element (ISRE) and mRNAs for ISGs, ISG56, and double-stranded RNA-dependent protein kinase (PKR) in HepaRG cells (Fig. 2H), suggesting that the anti-HBV activity is independent of ISG up-regulation. TNF␣, another cytokine that activates NF-B signaling (Fig. 2E, lower panels), also inhibited HBV infection (Fig. 2I). Thus, NF-B activation in host hepatocytes was critical for the anti-HBV activity of proinflammatory cytokines.

Early Phase of HBV Infection as Well as HBV Replication
Were Impaired by IL-1 Treatment-Although heparin, an attachment inhibitor, could block HBV infection only if added together with the HBV inoculum, pretreatment with IL-1␤ before HBV infection was sufficient to show anti-HBV activity (Fig. 3A, panel b). This activity was amplified by a prolonged treatment time of up to 12 h (Fig. 3B). Intriguingly, HBV cellular DNA was also reduced by IL-1␤ treatment following HBV infection (Fig. 3C, panel b). In contrast, IFN␣ was not effective by pretreatment (Figs. 3C, panel a, and 1A), although it did decrease HBV DNA by treatment after HBV infection (Fig. 3C,  panel b), consistent with previous reports that IFN␣ can sup-  press HBV replication (19,20,26). Thus, the anti-HBV activity of IL-1␤ is likely to be mechanistically different from that of IFN␣.
The HBV life cycle can be divided into at least two phases as follows: 1) the early phase of infection that includes attachment, entry, nuclear import, and cccDNA formation; and 2) the late phase representing HBV replication, including transcription, assembly, reverse transcription, DNA synthesis, and viral release (58). The early phase of HBV infection is not supported, but HBV DNAs persistently replicate in HepAD38 cells in the presence of tetracycline (38). IL-1␤ decreased the HBV DNA levels in HepAD38 cells (Fig. 3D), suggesting suppression of HBV replication. In addition, to examine the early phase preceding HBV replication, we infected HepaRG cells with HBV in the presence of IL-1␤ for 16 h and then immediately recovered cellular DNA in the trypsinized cells for quantification of HBV DNA (Fig. 3E). This procedure likely detected HBV DNA that had been internalized and evaded the host restriction before initiation of HBV replication because lamivudine showed no effect on the amount of DNA detected (Fig. 3E). In this experiment, IL-1␤ significantly decreased HBV DNA (Fig. 3E). cccDNA was also decreased by IL-1␤, suggesting that the early phase of HBV infection before cccDNA formation was also interrupted by IL-1␤.

IL-1 and TNF␣ Induced the Expression of AID-
The innate immune pathway against HBV infection remains largely unknown. Recently, accumulating evidence suggested that several APOBEC family proteins, especially A3G, suppressed HBV replication when overexpressed (27)(28)(29)(30)(31)(32)(33). In contrast, there was no report available suggesting the anti-HBV function of other restriction factors against HIV, TRIM5␣, tetherin/BST-2, and SAMHD1. We then investigated APOBEC family proteins as a candidate for an anti-HBV effector. The APOBEC family includes APOBEC1 (A1), A2, A3s, A4, and AID (59). Because some of these proteins are reported to be up-regulated in cytokine-stimulated hepatocytes (27,28,60,61), we examined the expression of these genes in cells treated with IL-1␤, TNF␣, and IFN␣ as a control for 12 h. The mRNA levels of A1, A2, and A3A were below the detection threshold. A3G and A3F mRNA were significantly expressed in HepaRG cells, and their expression levels were remarkably increased by IFN␣ treatment (Fig. 4A), as observed in other reports (27,28,61). IL-1␤ and TNF␣ did not significantly up-regulate A3s, and only AID was up-regulated 6 -10-fold by both cytokines (Fig. 4A). Induction of A3s by both IL-1␤ and TNF␣ was not observed at any time point examined until 12 h (data not shown). In contrast, induction of AID mRNA by IL-1␤ and TNF␣ was conserved in human hepatocyte cell lines, such as HepG2 and FLC4 cells, and in primary human hepatocytes (Fig. 4B). AID protein production was also increased in primary human hepatocytes by treatment with IL-1␤ and TNF␣ (Fig. 4C). This AID induction by IL-1␤ was suggested to be NF-B-dependent, because the up-regulation of AID mRNA was canceled by addition of NF-B inhibitors, Bay11-7082 or QNZ (Fig. 4D).
AID Played a Significant Role in the IL-1-mediated restriction of HBV-To examine the function of AID during HBV infection, we transduced AID ectopically into HepaRG cells using a lentiviral vector (Fig. 5A, left panel). The susceptibility of these AID-overexpressing cells to HBV was decreased by approximately one-third compared with the parental or empty vectortransduced HepaRG cells (Fig. 5A, right panel), suggesting that AID can restrict HBV infection. An AID mutant AID(M139V), with reported diminished activity to support class switching (48), also decreased the susceptibility to HBV infection, although the reduction in HBV susceptibility was moderate compared with the case of the wild type AID (Fig. 5B).
To examine the relevance of endogenous AID in the anti-HBV activity of IL-1, we transduced a lentiviral vector carrying a short hairpin RNA (shRNA) against AID (sh-AID) or a nonrelevant protein cyclophilin A (Fig. 5C), and we observed the anti-HBV activity of IL-1␤ in these cells. IL-1␤ decreased HBV infection in the control and sh-cyclophilin A -transduced cells by ϳ3.0-fold as determined by HBs secretion (Fig. 5D, lanes 1  and 2, black bars). In contrast, anti-HBV activity of IL-1␤ was limited to only 1.6 -1.7-fold in the cells transduced with sh-AIDs (Fig. 5D, lanes 3 and 4, black bars). Such relieved anti-HBV activity following AID knockdown was not observed in the case for heparin treatment (Fig. 5D, lanes 1-4, gray bars). Similar results were obtained by monitoring intracellular HBV DNA after infection (data not shown). Although the anti-HBV effect of IL-1␤ was not completely blunted, these data suggest that AID plays a significant role in mediating the anti-HBV effect of IL-1␤.
Similar observations were obtained in HBV-replicating cells overexpressing AID (Fig. 5, E and F). Core particle-associated HBV DNA in HepG2 cells transfected with an HBV-encoding plasmid was decreased by overexpression with AID as well as with A3G (Fig. 5E, lanes 1 and 3). Intriguingly, HBV DNA in core particles was also decreased by expression of an AID mutant AID(H56Y), which contains a mutation in the cytidine deaminase motif and is derived from a class switch deficiency patient (Fig. 5E, lane 2) (48). Southern blot also showed that the HBV rcDNA level in HepG2.2.15 cells was reduced by transduction with AID and another mutant AID(M139V), with diminished activity to support class switching (Fig. 5F) (48). These data suggest that AID could suppress HBV replication, and this restriction activity can be still observed with reduced enzymatic activity. In addition, AID was shown to interact with HBV core protein by coimmunoprecipitation assay (Fig. 5G). Moreover, overexpression of AID reduced the levels for nucleocapsid-associated HBV RNA (Fig. 5H). These results further suggest an antiviral activity of AID against HBV replication.
AID Could Induce Hypermutation of HBV DNA-Major enzymatic activity for APOBEC family proteins is the introduction of hypermutation in target DNA/RNA, and hypermutation accounts for antiviral activity for A3G against HIV-1 to some extent (2). Several groups reported that APOBEC family proteins could induce hypermutation in HBV DNA (27,30,32,34). Next we asked whether AID could induce hypermutations in HBV DNA. In differential DNA denaturation PCR analysis, a high content of A/T bases introduced by hypermutation decreased denaturation temperatures (51). As shown in Fig. 6A, ectopic expression of AID decreased the denaturation temperature of HBV DNA as shown by that of A3G. Sequence analyses of the HBV DNA X region amplified at 83°C by differential DNA denaturation PCR indicated a massive accumulation of G-to-A mutations by AID (Fig. 6B). The frequency of G-to-A mutations was augmented by AID expression (Fig. 6C). In this experiment, AID(JP8Bdel), a hyper-active mutant of AID (62), further promoted the accumulation of the G-to-A and C-to-T mutations, although AID(H56Y) showed mutations in HBV DNA equivalent with mock GFP control sample (Fig. 6C). Thus, AID had the potential to introduce hypermutation in nucleocapsid-associated HBV DNA.
IL-1 Suppressed the Infection of Different HBV Genotypes but Not That of HCV-We examined whether the antiviral activity of IL-1␤ and TNF␣ could be generalized to other viruses or was specific to HBV. As shown in Fig. 7A, the production of infec-tious HCV and HCV core proteins in the medium was not significantly altered by treatment with these cytokines in HCVinfected cells, compared to when IFN␣ was used as a positive control (Fig. 7A). In contrast, IL-1 suppressed the infection of HBV genotype A and C into HepaRG cells (Fig. 7B) as well as genotype D (Fig. 1C). These data suggest that the antiviral activity of proinflammatory cytokines IL-1 and TNF␣ is specific to HBV.

DISCUSSION
In this study, cytokine screening revealed that IL-1 and TNF␣ decreased the host cell susceptibility to HBV infection.  Right panels, these cells were infected with HBV followed by detection of secreted HBs protein as Fig. 1A. AID-transduced cells were less susceptible to HBV infection. C, HepaRG cells were transduced with lentiviral vector carrying shRNAs for AID (RG-shAID#1 and RG-shAID#2) or for cyclophilin A (RG-shCyPA) as a control. AID mRNA (left panel) and protein (right panel) were quantified by real time RT-PCR and immunoblot analysis. D, cells produced in C were infected with HBV in the absence or presence of IL-1␤ or heparin, and HBs was detected in the medium as in Fig. 1A to examine the anti-HBV effect of IL-1␤ and heparin. The fold reduction of HBV infection by IL-1␤ treatment is shown as IL-1␤ anti-HBV above the graph. The white, gray, and black bars indicate HBs value of the cells without treatment and with heparin and IL-1␤ treatment, respectively. The anti-HBV activity of IL-1␤ but not heparin was reduced in the AID-knockdown cells. E, AID and its mutant suppressed HBV replication. HepG2 cells were cotransfected with GFP-tagged AID, AID(H56Y), A3G, and GFP itself along with an HBV-encoding plasmid. Following 3 days, cytoplasmic nucleocapsid HBV DNA was quantified (upper graph), and the overexpressed proteins as well as actin were detected (lower panels). F, lentiviral vectors carrying AID, AID(M139V) mutant, A3G, or an empty vector (empty vector) were transduced or left untransduced (no transduction) into HepG2.2.15 cells. Nucleocapsid associated HBV DNA in these cells or in HepG2 cells (HBVϪ) was detected by Southern blot (upper panel). AID (middle panel) and A3G protein (lower panel) were also detected by immunoblot. G, HBV core interacted with AID. HepAD38 cells transduced without (no transduction) or with AID-expressing vector or the empty vector (empty vector) were lysed and treated with anti-core antibody (1st panel) or control normal IgG (2nd panel) for immunoprecipitation (IP). Total fraction without immunoprecipitation (3rd to 5th panels) was also recovered to detect AID (1st to 3rd panels), HBV core (5th panel), and actin (5th panel) by immunoblot. WB, Western blot. H, HBV RNA in core particles was extracted as shown under "Experimental Procedures" in HepG2 cells overexpressing HBV DNA together with or without AID or A3G.
This antiviral mechanism is rather unique, given that the intracellular immune response against viruses is typically triggered by IFNs. So far, type I, II, and III IFNs are reported to suppress the replication step of the HBV life cycle (19,20,25,26). In contrast, we suggest that IL-1 and TNF␣ inhibit the early phase of HBV infection as well as the replication. This is consistent with cumulative clinical evidence suggesting that these proinflammatory cytokines contribute to HBV elimination (63-65).
IL-1 and TNF␣ are generally produced mainly in macrophages and also in other cell types, including T cells and endothelial cells (66). Although the main producer cells of these cytokines in hepatitis B patients are not defined, it has been reported that the secretion of IL-1 and TNF␣ in nonparenchymal cells were increased by HBV infection into hepatocytes (67). TNF␣ production in macrophages was augmented by addition of recombinant HBc (68). A number of clinical studies cumulatively FIGURE 6. AID could induce hypermutation of HBV DNA. A and B, HepG2 cells were cotransfected with an expression vector for GFP-tagged AID, HA-tagged A3G, or GFP along with an HBV-encoding plasmid. 3 days after transfection, nucleocapsid-associated HBV DNA was extracted, and differential DNA denaturation PCR was performed to amplify the X gene segments. The numbers above the panels in A show denaturing temperatures. The X gene fragment amplified at 83°C in the AID sample was cloned in to a T vector and sequenced in B. Alignment of independent five clones with reference sequence (X02763) is indicated. C, AID and its mutant (JP8Bdel) induced G-to-A and C-to-T hypermutations in HBV DNA. HepG2 cells were transfected with expression vectors of GFP-tagged AID, AID(H56Y), AID(JP8Bdel), or GFP itself together with HBV encoding plasmid. Three days after transfection, cells were harvested, and nucleocapsidassociated HBV DNA was extracted. X gene fragments were amplified at 94°C and cloned in T vector. 55 clones were sequenced as described under "Experimental Procedures." The numbers indicate the clone numbers carrying the mutation. D, expression of GFP, GFP-tagged AID, AID(H56Y), and AID(JP8Bdel) is shown by immunoblot.
show that serum levels of IL-1 and TNF␣ are increased in hepatitis B patients (12). Recently, it has been a significant clinical problem that HBV reactivates during the course of treatment with immunosuppressants such as anti-TNF␣ agents (64,65). Taken together, it is proposed that acute or chronic HBV infection induces IL-1/TNF␣ from macrophages or other cells in the liver of infected patients, which can directly suppress HBV infection in hepatocytes, in addition to their immunomodulatory effects to the host immune cells. Although IL-1 level in HBV-infected patients varies between papers, Daniels et al. (63) reported that the peak IL-1␤ level in HBV-infected patients was 9 -36 ng/ml under Toll-like receptor stimulation, at which concentration IL-1␤ showed significant anti-HBV effects in this study. In general, downstream genes of NF-B include a number of antiviral factors such as viperin, iNOS, and RANTES (69). Although some of these genes may function cooperatively for IL-1-and TNF␣-induced anti-HBV machinery, our data suggest that AID, at least in part, plays a role in the elimination of HBV that was potentiated by proinflammatory cytokines IL-1 and TNF␣.
AID belongs to APOBEC family proteins that share enzyme activity to convert cytidine to uracil in mainly DNA, and occasionally RNA (51,70,71). Although AID was initially identified in B cells, chronic inflammation can trigger its expression in hepatocytes (60). The induction of AID was reportedly mediated by NF-B (60), consistent with the results in this study. Although AID in B cells is essential for class switch recombination and somatic hypermutation of immunoglobulin genes (70,72), the physiological role of AID in hepatocytes is unknown.
Although expression of AID in hepatocytes is still lower than in B cells, AID is reportedly expressed in the liver both in cell culture and in vivo settings (34,60). Our results raise the idea that AID plays a role in innate antiviral immunity. AID also has a role in virus-induced pathogenesis as it was reported to counteract oncogenesis induced by Abelson-murine leukemia virus (73). In addition, AID was reported to restrict L1 retrotransposition, which can predict the role of AID in innate immunity (74). This study is significant in that it revealed a biological function of AID in viral infection itself, linking it to the restriction of a pathogenic human virus. It will be interesting to analyze the role of AID in the infection process of other viruses in the future.
Although the mechanism for AID suppression of the HBV life cycle is the subject of future study, AID possibly targets the early phase of HBV infection, including entry as well as the replication stage, including assembly and reverse transcription (Fig. 3). It has been recently reported that chicken AID reduced cccDNA of duck HBV possibly through targeting cccDNA as well as nucleocapsid-associated HBV DNA (75). This study is likely to support the idea that AID may target cccDNA formed after HBV entry into hepatocytes, and also associates with nucleocapsid-associated HBV DNA during HBV replication, although it is not clear whether the innate immune machinery against HBV/duck HBV is conserved in human and chicken cells. A3G blocked HBV replication through the inhibition of reverse transcriptase (29), packaging of pregenomic RNA (33), and the destabilization of packaged pregenomic RNA (31) independently of its deaminase activity, and it also induced hypermutation of HBV DNA (27,30,32,34). It was recently reported that AID was packaged into the HBV nucleocapsid (51). Moreover, AID induced C-to-T and G-to-A hypermutations in HBV DNA/RNA, although the anti-HBV activity has not been demonstrated so far (51). The hypermutation activity of AID was likely to be dispensable for its anti-HBV replication function (Figs. 5 and 6), as reported for APOBEC3G by several groups (29,30,33). Further analysis is required to elucidate the precise mechanisms for AID-mediated suppression of the HBV life cycle.
In conclusion, we have identified that host cell susceptibility to HBV infection is modulated by IL-1 and TNF␣, and AID is involved in this machinery. This sheds new light on the link between proinflammatory cytokines and the development of the innate antiviral defense.