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Clinical and Diagnostic Laboratory Immunology, January 2005, p. 171-179, Vol. 12, No. 1
1071-412X/05/$08.00+0 doi:10.1128/CDLI.12.1.171-179.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Division of Infectious Diseases, The Children's Hospital of Philadelphia,1 Department of Pediatrics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania2
Received 2 October 2003/ Returned for modification 22 January 2004/ Accepted 4 November 2004
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In this study, we examined the evolution of mucosal virus-specific immunoglobulin A (IgA) and IgG secondary immune responses to determine the possible mechanisms by which RSV-specific memory B-cell responses are generated at the mucosal surface. The results from this and future studies may contribute to the development of an RSV vaccine.
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Virus. Human RSV strain Long (American Type Culture Collection, Manassas, Va.) was grown in Hep-2 cells (American Type Culture Collection). Supernatant fluids were clarified and titrated for infectivity by plaque assay as previously described (15). RSV was inactivated by incubation at 56°C for 30 min. Inactivated virus contained <10 PFU/ml.
Immunization of mice. Groups of five adult BALB/c mice were lightly anesthetized with ketamine (NLS Animal Health, Baltimore, Md.) and xylazine (NLS Animal Health). Mice were inoculated intranasally (i.n.) with 20 µl containing 9 x 104 PFU of RSV or comparable quantities of inactivated RSV (iRSV). Inoculations were performed with a micropipettor by repeated placement of small volumes of inoculum on nares until the entire volume had been inhaled. Control mice (five per group) were inoculated i.n. with 20 µl of Hep-2 cell medium (Eagles minimum essential medium [BioWhittaker, North Brunswick, N.J.], 10% fetal bovine serum [FBS; BioWhittaker], 1% HEPES [Gibco, Rockville, Md.], 1% L-glutamine [Gibco], 1% MEM essential vitamins [Gibco], penicillin G at 14 U/ml, and streptomycin at 14 l µg/ml [Gibco]).
Challenge of mice. Eight or 59 weeks after primary inoculation, five mice per group per time point were anesthetized as above and challenged i.n. with 20 µl containing 4 x 105 PFU of RSV strain Long.
Lymphoid organ cultures. To assess the production of RSV-specific antibodies by RALT, lymphoid organ cultures were established at various time points after challenge using a modification of previously published methods (1, 12). In brief, under sterile conditions organized nasal-associated lymphoid tissues (NALT) (as previously described [11]), cervical lymph nodes (CLN), and bronchial lymph nodes (BLN) were isolated. Following perfusion of the right cardiac ventricle with 3 ml of sterile phosphate-buffered saline (PBS), the right upper lobe of the lung was harvested. An approximately 4-mm tracheal segment, including the tracheal bifurcation, was isolated from each animal. Sublingual glands (SL), submandibular glands (SM), parotid glands (P), and palantine salivary glands (PSG) were harvested (3). All tissues were washed in Iscove's medium (CELLgro) containing 10% FBS and 0.1% gentamicin. Under a dissecting microscope (30x magnification), fat and connective tissue were removed from NALT, trachea, salivary glands, and lymph nodes. Four equivalent fragments were dissected from the harvested lung tissue. Each lung fragment, tracheal segment, BLN, CLN, SL, SM, P, PSG, or NALT was placed in an individual well of a 48-well plate (Costar Scientific, Braintree, Mass.) containing 0.5 ml of medium (Kennet's HY medium [Gibco], 100 µg of streptomycin [JRH, Lenexa, Kans.]/ml, 50 µg of gentamicin [Gibco]/ml, and 0.25 µg of amphotericin B [Fungizone; JRH]/ml). Samples were incubated at 37°C in a humidified atmosphere of 95% O2 and 5% CO2 for 5 days. Supernatant fluids were collected and tested for the presence of RSV-specific and total immunoglobulins by ELISA. Prior studies have demonstrated that the antibodies detected in lymphoid organ culture fluids are due to active production of antibodies by cultured tissues and not due to the passive transudation of serum-derived antibodies (2). We calculated the ratio of virus-specific to total antibodies detected in supernatant fluids to adjust for discrepancies in tissue size and viability.
Collection of mucosal secretions. An intravenous catheter with a 22-gauge 1-in. needle was inserted into the trachea, and 0.1 ml of sterile PBS was injected and retrieved. This bronchoalveolar lavage (BAL) specimen was collected and added to 0.9 ml of viral freezing medium (Eagles minimum essential medium, 5% FBS, 100 mM MgSO4, and 50 mM HEPES; pH 7.5). After decapitation and removal of the lower jaw, an intravenous catheter was inserted into the posterior nasopharynx. A 0.1-ml volume of sterile PBS was flushed through the nasal passage and collected from the nares into a 1.2-ml cryogenic vial containing 0.9 ml of viral freezing medium. BAL fluids and nasal wash samples were tested for the presence of RSV-specific and total immunoglobulins by ELISA, and viral titers were determined.
Isolation of lung tissue for viral quantification. A 0.5-ml volume of viral freezing medium was added to a 1.2-ml cryogenic vial and then weighed. After sacrifice, the right cardiac ventricle was perfused with 3 ml of sterile PBS. The left lobe of the lung was placed in a 1.2-ml cryogenic vial containing 0.5 ml of viral freezing medium and stored in liquid nitrogen.
Preparation of lung tissue for viral quantification. Lung samples were quickly thawed and weighed. For processing of the lung tissue, a sterile 7-ml Pyrex tissue grinder (Fisher Scientific) was used. One sample and 500 µl of Hep-2 medium were added to each grinder and mechanically homogenized. Samples were then diluted for viral quantification.
Quantification of infectious RSV. BAL, nasal washes, and processed lung samples were tested to determine the quantities of infectious RSV in each sample. Viral titrations were prepared in triplicate using subconfluent monolayers of Hep-2 cells in 12-well plates (Falcon) (15). The viral titers were determined by calculating the mean number of PFU in each set of triplicate wells. A positive control sample of known titer was plated with each assay.
Detection of RSV-specific and total immunoglobulins by ELISA.
Supernatant fluids, mucosal secretions, and sera were tested for the presence of RSV-specific total IgA and IgG antibodies. For detection of RSV-specific immunoglobulins, alternating wells of 96-well plates (Costar, Cambridge, Mass.) were coated with 105 BCH4 (a persistently RSV-infected fibroblast cell line) or BC (parental cell line) cells in 100-µl volumes (7). After overnight incubation in a humid chamber, plates were washed five times with PBS-0.05% Tween 20 (Sigma, St. Louis, Mo.), blocked with 300 µl of PBS containing 2% FBS-0.05% Tween (FBS-T), and incubated for 1 h at room temperature (RT). Wells were washed as described above, and 50 µl of supernatant fluids, mucosal secretions, or sera diluted in FBS-T was added to both positive and negative wells and incubated at RT for 1 h. After wells were washed, 50 µl of horseradish peroxidase-conjugated goat anti-mouse IgA or IgG (Southern Biotechnology Associates, Birmingham, Ala.) diluted 1:2,000 in FBS-T was added to the wells. Plates were incubated at RT for 1 h, and then wells were washed as described above. Equal volumes of TMB peroxidase substrate and peroxidase solution B (Kirkegaard & Perry Laboratories, Gaithersburg, Md.) were mixed, and 50 µl of this solution was added to each well. Plates were incubated for 5 min at RT, then 50 µl of 85% o-phosphoric acid (Fisher Scientific) was added to each well and, using a 450-nm filter on a microplate ELISA reader (Dynex Technologies, Chantilly, Va.), the optical density (OD) of each well was determined. Samples were considered positive if the mean OD value for RSV- or anti-immunoglobulin-coated wells was both
0.1 OD units and
2-fold the OD value for the corresponding negative control well. Quantities of total immunoglobulins were determined as previously described (1). Quantities of virus-specific and total immunoglobulins were determined using an isotype-specific standard curve that was constructed for each assay based on serial dilutions of purified mouse IgA and IgG (Sigma). Using the standard curve equations, threshold mean values were determined. A tissue sample was considered nonviable if no antibodies were detected in supernatant fluids by total immunoglobulin ELISA.
Statistical analysis. Continuous variables were expressed as means and were compared by use of the Student t test. Linear regression was used to control for time and compare the quantity of virus for the three groups. A two-tailed P value of <0.05 was considered statistically significant. Stata statistical software (Stata 8.0; Stata Corp.) was used for all calculations.
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FIG. 1. Eight and 59 weeks after primary i.n. immunization with live RSV, iRSV, or medium, five adult BALB/c mice per group per time point were challenged i.n. with 20 µl containing 4 x 105 PFU of RSV. Nasal washes, BAL fluid, and processed lung samples were tested to determine the quantities of PFU of RSV in each sample. Viral titrations were determined by plaque assay. Geometric means were calculated. *, P < 0.05 compared to quantities of virus detected in samples from control mice; all other differences were not statistically significant (P > 0.05).
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FIG. 2. Eight weeks after primary i.n. immunization with live RSV, iRSV, or medium, five adult BALB/c mice per group per time point were challenged i.n. with 20 µl containing 4 x 105 PFU of RSV. (A) Inductive tissues (NALT, CLN, and BLN) (A) and effector tissues (SM, tracheal lamina propria, and lung) (B) were isolated and cultured. The quantities of RSV-specific IgA produced were determined by ELISA 0, 3, 6, and 9 days after challenge. Data represent the arithmetic mean and standard error of the mean of the percentage of RSV-specific IgA compared to the total IgA produced. *, P < 0.05 compared to the percentage of RSV-specific IgA in samples from control mice; all other differences were not statistically significant (P > 0.05).
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FIG. 3. Eight weeks after primary i.n. immunization with live RSV, iRSV, or medium, five adult BALB/c mice per group per time point were challenged i.n. with 20 µl containing 4 x 105 PFU of RSV. Inductive tissues (NALT, CLN, and BLN) (A) and effector tissues (SM, tracheal lamina propria, and lung) (B) were isolated and cultured. The quantities of RSV-specific IgG produced were determined by ELISA 0, 3, 6, and 9 days after challenge. Data represent the arithmetic mean and standard error of the mean of the percentage of RSV-specific IgG compared to the total IgG produced. *, P < 0.05 compared to the percentage of RSV-specific IgG in samples from control mice; all other differences were not statistically significant (P > 0.05).
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FIG. 4. Eight weeks after primary i.n. immunization with live RSV, iRSV, or medium, five adult BALB/c mice per group per time point were challenged i.n. with 20 µl containing 4 x 105 PFU of RSV. Nasal washes, BAL fluid, and processed lung samples were tested to determine the quantities of RSV-specific IgA (A) or RSV-specific IgG (B) produced by ELISA 0, 3, 6, and 9 days after challenge. Graphed data represent the arithmetic mean and standard error of the mean of the percentage of RSV-specific antibodies compared to the total antibodies produced. Tables report the mean quantities of RSV-specific antibodies versus total antibodies.
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Primary immunization with live, not inactivated, RSV induced durable secondary mucosal humoral immune responses. At 59 weeks after primary immunization with live RSV, secondary mucosal RSV-specific IgA responses were 7- to 10-fold less than those observed at 8 weeks (Fig. 5). RSV-specific IgG was produced by all tissues tested 59 weeks after primary immunization with live RSV. However, RSV-specific IgG production was six- to eightfold less than that observed 8 weeks after primary inoculation (Fig. 6). No primary or secondary mucosal immune responses were observed in mice 1 year after primary immunization with iRSV.
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FIG. 5. At 59 weeks after primary i.n. immunization, lymphoid fragment cultures of four to five adult BALB/c mice per group were established. In addition, four to five previously immunized mice were challenged i.n. with 20 µl containing 4 x 105 PFU of RSV. Six days after challenge, lymphoid fragment cultures were established. The quantities of RSV-specific IgA produced were determined by ELISA 0 and 6 days after challenge. Data represent the arithmetic mean and standard error of the mean of the percentage of RSV-specific IgA compared to the total IgA produced.
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FIG. 6. At 59 weeks after primary i.n. immunization, lymphoid fragment cultures of four to five adult BALB/c mice per group were established. In addition, four to five previously immunized mice were challenged i.n. with 20 µl containing 4 x 105 PFU of RSV. Six days after challenge, lymphoid fragment cultures were established. The quantities of RSV-specific IgG produced were determined by ELISA 0 and 6 days after challenge. Data represent the arithmetic mean and standard error of the mean of the percentage of RSV-specific IgG compared to the total IgG produced.
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The relative contribution of RSV-specific IgA, compared to that of IgG, to protective immunity remains unclear. We found that complete protection of both the upper and lower airway was correlated with the presence of RSV-specific IgG in mucosal secretions at the time of challenge. While our studies demonstrated the active production of RSV-specific IgG by the effector tissues of the lower respiratory tract, the origins of mucosal RSV-specific IgG detected in nasal wash fluids remain unclear. Liang and coworkers found that the diffuse NALT of rodents was a site of prolonged production of antigen-specific antibodies (11). Although we found RSV-specific IgG was produced by SM B cells, we do not know whether antibodies produced by these tissues contribute to the protection of the nasal mucosa. In contrast to our findings, other investigators have demonstrated a critical role of antigen-specific IgA in RSV immunity. For example, Weltzin and colleagues found that mucosal IgA, if present at the time of challenge, could protect against upper and lower respiratory tract infection (16, 17). However, Fisher et al. demonstrated that mucosal RSV-specific IgA and IgG induced equivalent protection against viral replication (6). Our data suggest that RSV IgG, or another mucosal immunologic effector function present at the time of challenge, may mediate protection against viral replication within both the upper and lower airway. Similarly, investigators have found that parenteral administration of virus-specific IgG may reduce the titer of RSV in the respiratory secretions of infants infected with RSV. Other possible mediators of protection include cytotoxic T lymphocytes, antiviral cytokines, and innate immune factors.
In contrast to other investigators (8), we found that i.n. inoculation of mice with live RSV induced complete and durable protection against challenge. These observed differences might be related to our use of either a low challenge dose or a different viral strain than was used by some other investigators. Interestingly, we found that protection persisted for over 1 year in the absence of ongoing production of RSV-specific IgA by mucosal lymphoid tissues of the upper and lower airways. Conversely, RSV-specific IgG production persisted for 59 weeks after primary immunization with live RSV. Because epidemiological studies have demonstrated that repeated infections with RSV are common, our findings suggest that the murine model used for these studies is not a successful model for further study of human immune responses to primary RSV infections.
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