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Journal of Virology, May 2006, p. 4482-4490, Vol. 80, No. 9
0022-538X/06/$08.00+0 doi:10.1128/JVI.80.9.4482-4490.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Department of Veterinary Microbiology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan,1 Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan,2 Laboratory of Veterinary Infectious Diseases, Department of Applied Veterinary Science, Obihiro University of Agriculture and Veterinary Medicine, Inada-cho, Obihiro, Hokkaido 080-8555, Japan3
Received 10 November 2005/ Accepted 11 February 2006
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Feline calicivirus (FCV) is a member of the genus Vesivirus and causes upper respiratory tract disease and acute mouth ulceration, occasionally associated with chronic stomatitis and acute arthritis in cats (15, 21). It is comparatively easy to study the life cycle of FCV, because the virus replicates efficiently in cell culture without specific supplementation. Kreutz et al. (20) investigated the binding of FCV to feline and nonfeline cells. They reported that FCV bound to feline cells but also showed poor binding ability to nonfeline cells. It was also demonstrated that when nonfeline cells were transfected with genomic RNA of FCV, infectious virus could be recovered from the cells (20). These data suggest that FCV host specificity is restricted in the early stage of FCV infection in cells.
In this study, to elucidate the cellular determinant(s) of FCV host specificity, we applied an improved expression cloning method (28, 29) to identify cell surface molecules that interact with FCV. As a result, feline junctional adhesion molecule 1 (JAM-1) was identified as a binding receptor for FCV. JAM-1 is a member of the immunoglobulin (Ig) superfamily expressed by various cells and is involved in regulation of cell-cell interactions in the immune system and apical tight junction formation (10, 23). We here demonstrate that feline JAM-1 (fJAM-1) possesses the characteristics required of a functional receptor for FCV.
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TABLE 1. Cell lines used in this study
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1.5 x 104 cells) were mixed in 96-well plates and incubated at 37°C. Four wells were used for each dilution. Cells were observed for the cytopathic effect (CPE) for 3 days, and the 50% tissue culture infectious dose was calculated by the Behren-Kärber method. Virus titration was performed three times, and mean values were calculated. Statistical analysis was carried out by the Student t test. P values of <0.05 were considered statistically significant. For binding assays, these strains were propagated and concentrated as follows. CRFK cells grown in 690-cm2 roller bottles were infected with virus; the supernatant was harvested at 24-h postinfection, layered onto a 25% sucrose cushion, and ultracentrifuged at 120,000 x g for 2 h at 4°C. The pellet was suspended in phosphate-buffered saline (PBS) with Ca2+ and Mg2+. The suspension was centrifuged at 650 x g to remove insoluble matter and used for binding assays.
Expression cloning of fJAM-1.
Retrovirus-mediated expression cloning using a virus-coating dish was performed as described previously (28). A cDNA library was generated from poly(A)+ mRNA from the CRFK cell line in the retroviral vector pMX (19). The cDNA library was packaged in murine leukemia virus particles by transfection of Plat-E cells and used to infect P3U1 cells at a multiplicity of infection of
0.2. The dish, which was not tissue culture treated (Corning Glass Co., Corning, NY), was incubated at 4°C overnight with 10-µg/ml goat anti-mouse IgG (Rockland Immunochemicals, Gilbertsville, PA) in 4 ml of PBS. The dish was rinsed with PBS containing 2% FCS and incubated at 37°C for 30 min with an anti-FCV monoclonal antibody (MAb), 1D7 (35). The dish was washed four times with PBS containing 2% FCS. Ten milliliters of supernatant from CRFK cells infected with FCV F4 was applied to the antibody-coated dish and incubated at 20°C for 30 min. The medium was replaced by fresh viral fluid, and the incubation was repeated on three further occasions. At final replacement by fresh viral fluid, the dish was fixed with 0.1% paraformaldehyde at 20°C for 30 min. The dish was washed four times with PBS containing 2% FCS before the addition of 107 P3U1 cells transduced with the CRFK cDNA library in 4 ml PBS. The cells were incubated at 37°C for 2 h. Nonadherent cells were removed by being washed, and adherent cells were cultured for 5 days. Adherent cells formed colonies and were picked with a penicillin cup. From each of the clones, genomic DNA was isolated and subjected to amplification by PCR using LA Taq polymerase (TaKaRa, Tokyo, Japan) and pMX primers (18). Following amplification, one clone yielded a specific fragment, which was cloned in T vector (Invitrogen, Carlsbad, CA) and sequenced. The feline homolog of human JAM-1 (hJAM-1) was cloned into the BstXI site of pMX to produce pMX-fJAM-1. The fJAM-1 cDNA fragment was also cloned into the mammalian expression vector pcDNA3.1 (Invitrogen). pcDNA3.1-fJAM-1 was used for transfection of HmLu-1 cells and 293T cells using Mirus TransIT-LT1 (TaKaRa), and stably transduced HmLu-1 cells were selected with neomycin (Sigma). Cells transduced with empty pcDNA3.1 vector were used as a control.
Antibodies. Antibodies used in this study are listed in Table 2. The antisera against fJAM-1 were generated by DNA immunization. BALB/c mice (6 weeks old; female) were intramuscularly injected with 100 µg pcDNA3.1-fJAM-1 per head at 3-week intervals. Control sera were obtained from normal BALB/c mice (11 weeks old; female). Both sera were filtered through a sterile 0.2-µm filter (Gelman Sciences, Ann Arbor, MI).
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TABLE 2. Antibodies used in this study
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Virus-binding assay by flow cytometry. Cells were washed once in a flow cytometry buffer and incubated for 30 min on ice in 20 µl flow cytometry buffer containing concentrated FCV. In blocking experiments with the anti-fJAM-1 antibody or anti-hJAM-1 antibody, cells were incubated for 30 min on ice with the antibody before being subjected to the virus. The cells were washed twice and incubated for 20 min on ice with an anti-FCV MAb 2E1, which recognizes an epitope on the capsid protein (unpublished data). The cells were washed twice and resuspended in 5 µl flow cytometry buffer containing 10-fold dilutions of FITC-conjugated anti-mouse Ig and incubated for 20 min on ice. For the blocking experiment with the anti-fJAM-1 antibody, the second antibody was not required because the MAb 2E1 was labeled with Alexa Fluor 488 (Zenon, Oakville, Ontario, Canada), according to the manufacturer's instructions. The cells were washed and analyzed with a FACSCalibur.
Infection of fJAM-1-transfected HmLu-1 or 293T cells. HmLu-1 cells stably transduced and 293T cells transiently transduced with pcDNA3.1-fJAM-1 or pcDNA3.1 only were inoculated with FCV at multiplicity of infection of 10, incubated at 37°C for 1.5 h, and washed with DMEM. Cells were harvested and fixed with ice-cold acetone 1, 12, 24, and 48 h after inoculation; and intracellular viral proteins were detected by indirect immunofluorescence assay (IFA) using an anti-FCV MAb, 8G1 (30). At 24 h postinfection, the supernatants of HmLu-1 cells transduced with pcDNA3.1-fJAM-1 or pcDNA3.1 were collected, and viral titers were measured as described above.
Inhibition experiments. CRFK or Vero cells (1.5 x 104) per well in 96-well plates were plated 1 day before infection. CRFK cells were washed once with PBS and incubated with 50 µl anti-fJAM-1 antibody or control mouse serum at fivefold dilutions, and Vero cells were washed and incubated with 50 µl (100-µg/ml) anti-human JAM-1 antibody or normal goat IgG for 1 h at 37°C. After antibodies were removed, CRFK cells were inoculated with FCV at a multiplicity of infection of 0.1 and incubated for 1 h at 37°C. Vero cells were inoculated with the viruses at a multiplicity of infection of 1 and incubated under the same conditions. The inocula were removed, CRFK cells were monitored for CPE for 2 days, and Vero cells were monitored for CPE for 4 days. At 0 h and 2 days (CRFK cells) or 4 days (Vero cells) postinoculation, cell supernatants were collected and virus titration was performed as described above.
PCR cloning of simian JAM-1 cDNA from Vero cells.
Total RNA was isolated from Vero cells using an SV Total RNA Isolation system (Promega, Madison, WI), and first-strand cDNA was synthesized by SuperScript III reverse transcriptase (Invitrogen) and an oligo(dT) primer. Primers SCF (5'ATCGCGATGGGGACAAAGGC) and SCR (5'AGGCTCACACCAGGCATGAC) were designed according to the sequences of feline and human JAM-1 to identify simian JAM-1 (sJAM-1). An
900-bp DNA fragment was amplified by PCR using LA Taq polymerase (TaKaRa), and the sequence was determined.
Nucleotide sequence accession numbers. Nucleotide sequences of fJAM-1 and sJAM-1 are available from the DNA Data Bank of Japan (DDBJ), accession numbers AB196140 and AB196141, respectively.
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1.5 kb was amplified by PCR with pMX primers. Sequence analysis revealed that the amplified insert encoded the feline homolog of JAM-1. The nucleotide and deduced amino acid sequences of fJAM-1 cDNA (DDBJ accession number AB196140) are shown in Fig. 1. Reproducible binding to FCV F4 coated on dishes was observed when the cDNA of fJAM-1 was expressed in P3U1 cells (data not shown).
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FIG. 1. Nucleotide and deduced amino acid sequences of fJAM-1 cDNA. The putative hydrophobic signal peptide and transmembrane sequences are underlined. Four cystein residues that are predicted to form Ig-like domains are boxed. The putative N-linked glycosylation site is marked in boldface and underlined.
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FIG. 2. Detection of JAM-1 molecules on the cell surface by flow cytometry using anti-JAM-1 antibody. Cells used in this assay were Vero, 293T, CRFK, and HmLu-1 cells stably transduced with pcDNA3.1-fJAM-1 (HmLu-1-fJAM-1) or vector alone (HmLu-1-pcDNA). Cells were incubated with anti-JAM-1 antibody (shaded area), control antibody (thick line), or buffer only (thin line). Cells were incubated with a second antibody and analyzed by flow cytometry. (A) Detection of JAM-1 on cells using anti-fJAM-1 antibody. (B) Detection of JAM-1 on cells using anti-hJAM-1 antibody.
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FIG. 3. FCV-binding assay by flow cytometry. HmLu-1-fJAM-1 or HmLu-1-pcDNA cells were incubated with four strains of FCV (thick line) or with buffer only (thin line). Viral antigens on cells were detected with an anti-capsid MAb by flow cytometry.
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FIG. 4. Binding inhibition assay by flow cytometry using anti-fJAM-1 and anti-hJAM-1 antibodies. (A) CRFK cells were incubated with anti-fJAM-1 antibody (shaded area) or control normal mouse serum (thick line) prior to incubation with FCV. In parallel, cells were incubated with buffer only (thin line). Vero (B) and 293T (C) cells were preincubated with anti-hJAM-1 antibody (shaded area) or normal goat IgG (thick line). FCV capsids were detected on the cells by flow cytometry.
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fJAM-1 expression confers FCV susceptibility on nonpermissive cells. We investigated whether fJAM-1 expression could mediate FCV infection. fJAM-1-expressing HmLu-1 or 293T cells were inoculated with FCV F4 at a multiplicity of infection of 10; after the cells were washed with DMEM, viral antigens in the cells were examined by IFA at 1, 12, 24, and 48 h postinoculation. As shown in Fig. 5A, viral antigens were detected as early as 12 h postinoculation in HmLu-1-fJAM-1 cells. No viral capsid protein was detected in FCV-inoculated control HmLu-1-pcDNA cells. fJAM-1-expressing 293T cells showed similar results (Fig. 5B). When we used FCV F9, F25, and 91-40 for infection, results were same as for the F4 strain (data not shown). Furthermore, viral titers in supernatants of HmLu-1-fJAM-1 cells at 24 h postinfection were 102- to 103-fold higher than those of control cells (Fig. 5C). These data suggest that ectopic expression of fJAM-1 in nonpermissive cells renders them susceptible to infection with FCV.
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FIG.5. FCV infection in nonpermissive cells expressing fJAM-1. (A) HmLu-1-fJAM-1 or HmLu-1-pcDNA cells were inoculated with FCV F4 at a multiplicity of infection of 10 and fixed with acetone at 1, 12, 24, and 48 h postinfection. FCV antigens were detected by IFA. (B) 293T cells transduced with pcDNA3.1-fJAM-1 (293T-fJAM-1) or vector alone (293T-pcDNA) were inoculated with FCV F4 and fixed with acetone at 24 h postinfection. FCV antigens were detected by IFA. (C) Viral titers in supernatants of HmLu-1-fJAM-1 and HmLu-1-pcDNA cells at 24 h postinfection. Viral titers are expressed as the mean of three independent experiments. *, P < 0.05, and **, P < 0.01, compared to the supernatants of control cells.
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FIG. 6. Effect of anti-fJAM-1 antibody on FCV infection in CRFK cells. CRFK cells were incubated with the anti-fJAM-1 antibody or control normal mouse serum prior to inoculation with FCV at a multiplicity of infection of 0.1. After antibody and inocula were removed, cells were added to fresh medium and incubated for 0 h or 2 days. Cells were observed, and virus titration was performed. (A) CRFK cells at 2 days postinfection or mock infection. (B) Viral titers in supernatants of CRFK cells at 0 h or 2 days postinfection. Viral titers are expressed as the mean of three independent experiments. **, P < 0.01, compared to the supernatants of CRFK cells incubated with anti-fJAM-1 antibody at 2 days postinfection.
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FIG. 7. Effect of anti-hJAM-1 antibody on FCV infection in Vero cells. Vero cells were incubated with anti-hJAM-1 antibody or normal goat IgG prior to inoculation with FCV F4 and F25 strains at a multiplicity of infection of 1. After antibody and inocula were removed, cells were added to fresh medium and incubated for 0 h or 4 days. Cells were observed, and virus titration was performed. (A) Vero cells at 4 days postinfection or mock infection. (B) Viral titers in supernatants of Vero cells at 0 h or 4 days postinfection. Viral titers are expressed as the mean of three independent experiments. **, P < 0.01, compared to the supernatants of Vero cells incubated with anti-hJAM-1 antibody at 4 days postinfection.
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JAM-1 is a member of the Ig superfamily and is expressed in various tissues and cell types, including leukocytes and platelets, as well as endothelial and epithelial cells, and is specifically localized at apical tight junctions (10, 23). However, FCV locally infects the respiratory tract, except for strains that cause virulent systemic disease (16, 27), suggesting that JAM-1 is a critical determinant of FCV host specificity and that tissue-specific molecular components other than JAM-1 may result in local infection of FCV in vivo. Consistent with this hypothesis, CRFK cells treated with neuraminidase, followed by O-glycanase treatment, demonstrated decreased FCV-binding ability (20). Therefore, such carbohydrates expressed in specific tissues or cells may enhance JAM-1-mediated FCV infection. Of course, it is possible that FCV uses an alternative receptor in vivo besides JAM-1, which has a tissue-specific distribution.
The majority of FCV strains, as represented by F9 and 91-40, have no growth ability in Vero cells, but binding to the cells was detected (Fig. 4B). When Vero cells are transfected with genomic RNA from these viruses, infectious virus can be recovered from Vero cells (20). Hence, these strains may be restricted in Vero cells at the early stage, probably after attachment, such as penetration and uncoating. The full genome sequences of F4 (infectious in Vero cells) and F9 (noninfectious in Vero cells) have been determined (5, 26, 36). Studies using infectious cDNA clones of FCV F4 and F9 strains would clarify the difference between F4 and F9 in terms of growth ability in Vero cells.
As JAM-1 is highly conserved among cats, monkeys, and humans (Fig. 8), it is possible that FCV binds to JAM-1 in these three animals via a conserved region(s). Although FCV has no growth ability in 293T cells, it seems unlikely that 293T cells have a postentry resistance mechanism against FCV, because ectopic expression of fJAM-1 in 293T cells supported FCV infection (Fig. 5B). Therefore, hJAM-1, at least expressed on 293T cells, can be recognized by FCV but cannot support virus infectivity. However, we found two amino acid residues conserved between fJAM-1 and sJAM-1 in extracellular domains of JAM-1, i.e., S91 and K155 in fJAM-1, but not in hJAM-1 (Fig. 8). The two residues in fJAM-1 may play an important role in FCV infectivity. Further studies are required to determine at which stage(s) the strains of FCV that cannot replicate in Vero and 293T cells are restricted after attachment. There has been no report in the past that FCV has the ability to infect monkeys or humans, suggesting that unidentified factors in addition to receptors may affect host species restriction.
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FIG. 8. A comparison of JAM-1 amino acid sequences of cats, monkeys, and humans. The putative signal peptide and transmembrane domain are underlined. Simian and human JAM-1 had a high identity of 96.7%, while fJAM-1 had 75.6% sequence identity with hJAM-1 and 76.6% sequence identity with sJAM-1. Two amino acid residues (S91 and K155) in extracellular domains of JAM-1 that were conserved between cats and monkeys, but not in humans, are boxed.
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B and apoptosis, and receptor binding and disassembly of the viruses are required to elicit apoptosis (8, 11). However, the attachment and internalization of FCV do not induce apoptosis in cells. Instead, viral replication and protein synthesis trigger caspase activation and apoptosis (1, 31). Thus, the stages of FCV infection eliciting an apoptotic response are thought to be distinct from those of reovirus. As well as JAM-1, coxsackievirus-adenovirus receptor and nectin, which are components of the apical tight junction complex of Ig-like proteins, regulate tight junctions (7, 14, 23). Some viruses are reported to use these Ig-like proteins as receptors. Adenovirus and coxsackieviruses use the coxsackievirus-adenovirus receptor (3); alphaherpeviruses and poliovirus use the nectin family (12, 34) for host cell entry. Targeting Ig-like proteins of apical tight junction complexes in infection may support efficient propagation in vitro, and possibly in vivo. Studies of FCV interaction with JAM-1 molecules may help determine how the viruses invade tight junction barriers of mucosal membranes.
This work was supported by grants from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
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