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Journal of Virology, March 2002, p. 2403-2409, Vol. 76, No. 5
0022-538X/02/$04.00+0 DOI: 10.1128/jvi.76.5.2403-2409.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Department of Pediatrics, Program in Gene Therapy, The University of Iowa College of Medicine, Iowa City, Iowa 52242 ,1 Molecular Medicine Program, Mayo Clinic, Rochester, Minnesota 559052
Received 5 September 2001/ Accepted 13 November 2001
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Previous studies using monkey kidney cells (Vero C1008) and human colon carcinoma cells (Caco-2) suggested that MV preferentially enters polarized epithelial cells across the apical surface (2, 3). Additionally, studies using Vero C1008, Caco-2, and Madin-Darby canine kidney (MDCK) cells demonstrated that MV is released from the apical membrane of polarized epithelia (2, 3, 15, 18). Indeed, because MV is spread via an aerosol route, it would also be hypothesized to enter the apical surface of polarized airway epithelia. However, not all polarized epithelia behave in the same way, and the airways in particular have evolved barriers to viral infection (25).
The apical surface of airway epithelia presents a formidable barrier to many viral vector systems currently under investigation for pulmonary gene therapy applications (29). To develop a lentivirus vector capable of transducing the apical surface of airway epithelia, we are investigating the pseudotyping of lentivirus vectors with envelope glycoproteins from respiratory viruses that naturally enter via the apical surface of airway epithelia. We hypothesized that the MV envelope glycoproteins (H and F) were excellent pseudotyping candidates for multiple reasons. As mentioned above, previous in vitro evidence demonstrated that MV preferentially enters polarized epithelia across the apical surface. In addition, the MV viral receptors are known and reagents to localize those receptors are readily available (7, 24). However, before pseudotyping experiments were performed, we sought to verify that MV would transduce efficiently when applied to the apical surface of primary cultures of well-differentiated human airway epithelia.
To evaluate the polarity of MV entry in human airway epithelia, we utilized a recombinant MV vaccine strain expressing the enhanced green fluorescent protein (MV-eGFP) (8). Previously this recombinant, replication-competent virus was used to monitor viral replication, in real time, in animal tissues (9). In this study, we report the unexpected observation that MV enters more efficiently from the basolateral surface than from the apical surface of differentiated primary cultures of human airway epithelial cells. In addition, the receptor for the vaccine strain of MV, CD46, was abundantly expressed in the airway cells and largely polarized to the apical surface as determined by confocal microscopy. These data suggest that the MV may require access to the basolateral surface of airway epithelia in vivo for efficient infection to occur. In addition, receptor localization may not be the limiting barrier to MV infection.
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·cm2). This study was approved by the institutional review board of the University of Iowa.
Caco-2 cells were seeded onto the same semipermeable membranes as were the airway epithelial cells. Caco-2 cells were maintained with USG medium applied to both the apical and basolateral surfaces and incubated at 37°C with 5% CO2. The transepithelial resistance of all Caco-2 samples was >400
·cm2 prior to infection with MV-eGFP.
Virus production and titer determination. MV-eGFP production was conducted as previously described (8). Briefly, MV-eGFP was produced in Vero cells cultured in Dulbecco's modified Eagle's medium (Gibco) containing 8% newborn calf serum (Gibco) and penicillin-streptomycin (100 µg/ml). MV-eGFP titers of approximately 107 50% tissue culture infective doses/ml were obtained. To determine the polarity of viral release from airway epithelia, 500-µl washings were collected from both the apical and basolateral surfaces of airway epithelia at progressing time points. The time points included preinfection and 1, 3, 5, and 7 days post-MV-eGFP infection. Transepithelial resistances were concurrently measured at the time points of wash collection. Washings were stored at -80°C until they were collectively thawed and used to infect Vero cells at a limiting dilution. Titers were calculated by counting eGFP-positive cells after 30 h.
Infection of polarized airway epithelial cells. MV-eGFP viral preparations were diluted in USG medium, and 100 µl of the solution was applied to the apical surface of airway epithelial cells (multiplicity of infection [MOI] of 1). After incubation for 4 h at 37°C, the virus was removed and cells were further incubated at 37°C, for indicated time periods. To infect airway epithelia with MV-eGFP from the basolateral side, the Millicell culture insert containing the airway epithelial culture was turned over and the virus was applied to the basolateral surface for 4 h in 100 µl of USG medium. Following the 4-h infection, the virus was removed and the culture was turned upright and allowed to incubate at 37°C with 5% CO2 for the indicated time periods. A recombinant adenovirus vector expressing eGFP (Ad5-eGFP), produced as previously described (1), was used as a positive control.
Western blot analysis. Western blot analysis for verifying CD46 protein expression was conducted using standard techniques. Briefly, cell lysates were denatured for 5 min at 100°C in Laemmli sample buffer, electrophoresed on previously prepared 10% polyacrylamide gels (Bio-Rad; catalog no. 161-1155) at 125 V, and transferred to pure nitrocellulose (Bio-Rad; catalog no. 162-0145) overnight at 200 mA. The membrane was probed with a rabbit anti-human CD46 primary antibody (courtesy of J. Atkinson, St. Louis, Mo.) at a 1:4,000 dilution and detected using goat anti-rabbit immunoglobulin G conjugated to alkaline phosphatase at a 1:1,000 dilution (Sigma; catalog no. A-3812).
Immunohistochemistry and confocal microscopy. Epithelial cells were washed with 1x phosphate-buffered saline (PBS), fixed in 2% paraformaldehyde for 5 to 10 min, and rinsed with 1x PBS. The epithelial cells were then incubated for 30 min at 37°C with a rabbit anti-human CD46 antibody (tail2) (4) diluted 1:100 in Hanks' buffer (Gibco). The cells were washed with 1x PBS and incubated with a tetramethyl rhodamine isocyanate (TRITC)-conjugated anti-rabbit 2° antibody (Sigma; T6778) diluted 1:100 in 1x PBS for 30 min at 37°C. The primary and secondary antibodies were always applied to both the apical and basolateral surfaces. Tracheal tissues were paraffin embedded using standard techniques, sectioned, and deparaffinized prior to immunostaining. Immunohistochemistry assays of sectioned tracheal specimens were performed using the same protocol as previously described for the epithelial cells. Images were captured with a Bio-Rad MRC-1024 Hercules laser scanning confocal microscope equipped with a Kr-Ar laser.
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FIG. 1. Polarity of MV infection in well-differentiated human airway epithelia. The apical or basolateral surfaces of human airway epithelia were infected with MV-eGFP at an MOI of 1 for 4 h and allowed to incubate for 2 or 7 days as indicated. Panels I to L are representative vertical sections of panels E to H, respectively. Arrowheads indicate uninfected basal cells. Bars = 100 µm (A to D) or 50 µm (E to L). n = 6.
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FIG. 2. Polarity of MV infection in Caco-2 cells. Polarized Caco-2 cells were infected with MV-eGFP for 4 h and allowed to incubate for 2 days. MV-eGFP was applied at an MOI of 1 in a 100-µl volume to the apical surface (A), at an MOI of 3 in a 300-µl volume to the basolateral surface via the basal medium (B), or at an MOI of 1 in a 100-µl volume directly to the basolateral surface by inverting the support membrane (C). Caco-2 cells were immunostained with a rabbit anti-human CD46 primary antibody and a goat anti-rabbit TRITC secondary antibody (D and E). Images were captured by confocal microscopy. A representative vertical section is shown (E). Bar = 50 µm. n = 3.
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FIG. 3. Time course of MV-eGFP infection. MV-eGFP was applied to the apical surface of airway epithelia at an MOI of 1 for 4 h and allowed to incubate for the indicated time periods. Focus appearance and disappearance were documented over the time interval indicated by using an inverted fluorescence microscope. Though uncommon, groupings of foci were documented to ensure that the same field was observed over time, as indicated by arrowheads. n = 3.
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FIG. 4. Maintenance of epithelial integrity following MV-eGFP infection. The epithelial integrity was determined by ohmmeter measurement of the transepithelial resistance. Values were corrected for the blank filter resistance and further standardized against baseline readings and uninfected counterparts. Neither corrected nor raw numbers resulted in a statistically significant variation from uninfected epithelia. Error bars represent the standard errors of the means. n = 6.
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FIG. 5. Polarity of MV-eGFP release. Human airway epithelia were infected with MV-eGFP at the apical or basolateral surface as described in Materials and Methods. Both apical and basolateral washings were collected over the time interval indicated, including a preinfection baseline washing. Titers of washings were determined on Vero cells. Error bars represent the standard errors of the means. n = 6.
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FIG. 6. CD46 expression in human airway epithelia. Human airway epithelial cells were immunostained with a rabbit anti-human CD46 primary antibody and a goat anti-rabbit TRITC-conjugated secondary antibody (A). Images were captured using confocal microscopy. A representative vertical section is shown (B). Bar = 50 µm. Western blot analysis of human airway epithelial cell (hAEC) lysate and human tracheal (hT) lysate was conducted with a rabbit anti-human CD46 polyclonal antibody as described in Materials and Methods (C).
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66-kDa isoform (named BC) contains a heavily O-glycosylated region that the smaller isoform (named C) lacks (14). The BC and C isoforms may each have one of two different cytoplasmic tails (named 1 and 2) which are not easily distinguishable by Western analysis. Under nonreducing conditions, we observed both BC and C isoforms in human tracheal protein extracts as well as in human airway epithelial cell protein extracts (Fig. 6C). In addition, Western analysis of CD46 expression in Vero cell lysates detected both isoforms (data not shown). These results are consistent with previous CD46 Western analysis of human lung tissue (17) and suggest that primary cultures of human airway epithelial cells express multiple isoforms of CD46. To confirm the presence and expression pattern of CD46 in vivo, human tracheal specimens were studied by immunofluorescence microscopy. Hematoxylin and eosin staining of the tracheal cross sections reveal tissue morphology and the presence of ciliated epithelial cells (Fig. 7A, black arrows). As shown in the subsequent serial section (Fig. 7B), CD46 expression was detected in tracheal epithelia and more intensely at the apical surface of ciliated cells (white arrows). When normal rabbit serum was used in place of the polyclonal CD46 primary antibody, only a diffuse nonspecific background immunofluorescence was detected (data not shown). The observation of CD46 localization in the epithelia of tracheal tissue sections closely mirrors our observation of CD46 expression and localization in vitro (Fig. 6).
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FIG. 7. CD46 expression in human tracheal sections. Human tracheal explants were fixed, mounted in paraffin blocks, and sectioned. Hematoxylin and eosin staining reveals the cellular morphology of the airway epithelia (A). Alternating sections of the tissue were immunostained with a rabbit anti-human CD46 primary antibody and a goat anti-rabbit TRITC-conjugated secondary antibody as described in Materials and Methods (B). Arrows indicate ciliated airway epithelial cells.
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Interestingly, we found that, though the virus preferentially enters across the basolateral membrane, it is released predominantly from the apical surface. We can envision a model in which MV is brought into contact with the airways via inhalation. Infection then occurs at sites of epithelial injury or to a lesser extent across the apical surface. Once replication has occurred in the epithelia, virus is released back into the lumen where it can be taken up by macrophages or released back into the environment in respiratory droplets. Macrophages may ultimately be responsible for taking the virus to the bloodstream, after which the systemic manifestations of the viral infection begin.
As discussed, our data suggest that MV preferentially enters the airway epithelium at the basolateral surface and is released at the apical surface. These data suggest a favored basolateral-to-apical movement of the virus that could help to explain the apparent resistance of airway epithelia to secondary infection as seen in Fig. 3. Interestingly, when MV is applied to the basolateral surface and the pattern of eGFP expression is visually documented over time, the foci of expression are much more numerous than, but behave in a similar manner as, those in an apical infection (data not shown). Individual foci initially become larger and more fluorescently intense but eventually wane in size and intensity; furthermore, transepithelial resistance remains relatively constant (Fig. 4).
Another potential explanation for the failure of secondary infection to progress in airway epithelia concerns the phenomenon of CD46 down-regulation following MV infection (11, 19, 23). CD46 immunostaining of airway epithelia following apical MV infection revealed prevalent CD46 expression across the epithelial sheet but not within areas of eGFP-positive cells (data not shown). These data suggest that local down-regulation of CD46 occurs within sites of infection, perhaps as a means to prevent superinfection. Down-regulation was not observed in neighboring uninfected cells; however, such conclusions require a quantitative analysis of CD46 levels before and after MV infection by fluorescence-activated cell sorting or Western analysis.
A particularly puzzling aspect of this work is the finding that the virus preferentially enters via the basolateral membrane, while CD46 expression is highest at the apical surface. One possible explanation for this observation stems from the notion that receptor accessibility is not the only requirement for successful viral infection. As in vivo, the primary cultures of human airway epithelia secrete airway surface liquid and mucus and are heavily ciliated at the apical surface (30). These features may provide barriers to infection at the apical surface. In addition, the apical surface has a dense glycocalyx that is not present at the basolateral surface and may act as a physical barrier for some viruses (20). An alternate explanation for the poor apical transduction efficiency is the absence of a putative coreceptor from the apical surface. This hypothetical coreceptor may be strictly polarized to the basolateral surface.
It is important to mention that other respiratory viruses transduce the apical surface of polarized epithelia with equal or greater efficiency than they do the basolateral surface (6). For example, we previously reported that human coronavirus (serotype 229E) efficiently transduces the apical surface of well-differentiated airway epithelia (28). In addition, respiratory syncytial virus and parainfluenza virus also have the capacity to enter polarized human airway epithelial cells through the apical surface (P. L. Sinn and P. B. McCray, unpublished observations). Therefore, these cells are not inherently impenetrable to all enveloped viruses at the apical surface. Conversely, MV is not the first respiratory virus to be shown to preferentially transduce the basolateral surface. For example, adenovirus serotypes 2 and 5 preferentially transduce the basolateral surface of airway epithelia, although the original assumption was that they would cross at the apical surface (20, 26). However, unlike MV, the receptor for adenovirus serotypes 2 and 5 (coxsackievirus and adenovirus receptor) is polarized to the basolateral surface.
The pulmonary epithelium has evolved strategies to prevent the host invasion of microbes and viruses (25). Indeed, multiple factors present on the apical surface of airway epithelia may preclude viral transduction. Airway surface epithelia secrete antimicrobial proteins, peptides, and interferons; thus, innate host defenses may impact the ability of MV to infect from the apical surface of airway epithelia. Our data suggest that, in vivo, MV requires access to the basolateral surface of airway epithelia for efficient viral infection to occur. In addition, the limiting barrier to MV infection in the conducting airways of respiratory epithelia is not CD46 receptor expression on the apical surface. These results imply that pseudotyping recombinant lentivirus with Edmonston strain MV envelope glycoproteins to target the apical surface of airway epithelia is likely to be an inefficient strategy.
We acknowledge the support of the Cell Culture Core and Cell Morphology Core, partially supported by the Cystic Fibrosis Foundation, NHLBI (PPG HL-51670), and the Center for Gene Therapy for Cystic Fibrosis (NIH P30 DK-54759). This work was supported by NIH grant RO1 HL-61460 (P.B.M.), PPG grant HL-51670 (P.B.M.), the Cystic Fibrosis Foundation, and the Mayo and Siebens Foundations. P.L.S. was supported by an institutional NRSA training grant (HL-07121).
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