ABSTRACT
The genus Bordetella includes a group of closely related mammalian pathogens that cause a variety of respiratory diseases in a long list of animals (B. bronchiseptica) and whooping cough in humans (B. pertussis and B. parapertussis). While past research has examined how these pathogens are eliminated from the lower respiratory tract, the host factors that control and/or clear the bordetellae from the upper respiratory tract remain unclear. We hypothesized that immunoglobulin A (IgA), the predominant mucosal antibody isotype, would have a protective role against these mucosal pathogens. IgA−/− mice were indistinguishable from wild-type mice in their control and clearance of B. pertussis or B. parapertussis, suggesting that IgA is not crucial to immunity to these organisms. However, naïve and convalescent IgA−/− mice were defective in reducing the numbers of B. bronchiseptica in the upper respiratory tract compared to wild-type controls. Passively transferred serum from convalescent IgA−/− mice was not as effective as serum from convalescent wild-type mice in clearing this pathogen from the tracheae of naive recipient mice. IgA induced by B. bronchiseptica infection predominantly recognized lipopolysaccharide-containing O-antigen, and antibodies against O-antigen were important to bacterial clearance from the trachea. Since an IgA response contributes to the control of B. bronchiseptica infection of the upper respiratory tract, immunization strategies aimed at inducing B. bronchiseptica-specific IgA may be beneficial to preventing the spread of this bacterium among domestic animal populations.
Clinical and experimental findings show that individuals deficient in the immunoglobulin A (IgA) isotype of antibodies, the main immunoglobulin in mucus secretions, are more susceptible to certain sinopulmonary infections (17, 34, 36). In animal studies, IgA is important to immunity against gram-positive bacteria, such as Streptococcus pneumoniae (19, 39). These antibodies also effectively reduced the nasal carriage of influenza virus in a mouse model (34). However, there are conflicting reports regarding the contribution of IgA to immunity against gram-negative bacterial pathogens (1, 17, 18, 22).
This study analyzes the protective role of IgA in response to infection by the gram-negative mammalian respiratory pathogens Bordetella bronchiseptica, Bordetella pertussis, and Bordetella parapertussis. B. bronchiseptica infects a wide range of mammals and is an etiologic agent of kennel cough in dogs, atrophic rhinitis in pigs, and snuffles in rabbits (4, 23). B. pertussis and B. parapertussis are human-adapted pathogens that colonize the respiratory tract, causing whooping cough (6, 25). Since these three closely related bacterial species colonize the respiratory mucosa, we hypothesized that Bordetella-specific IgA would be important to their elimination from the respiratory tract.
Previously, we showed that B cells are required for clearance of B. bronchiseptica, B. pertussis, and B. parapertussis from the murine respiratory tract, but passively transferred serum antibodies clear these pathogens only from the lower respiratory tract (20, 21, 42). Interestingly, passively transferred serum antibodies have little, if any, effect on these species in the upper respiratory tract (20, 21, 42). Since the passively transferred serum contained low levels of IgA (20, 42), we hypothesized that IgA may be important in controlling these bacteria in the upper respiratory tract.
Here we demonstrate that IgA is essential for controlling B. bronchiseptica in the upper respiratory tract of both naïve and immunized hosts. Additionally, a passive transfer of serum from convalescent wild-type mice reduces B. bronchiseptica numbers in the trachea more effectively than serum from convalescent IgA−/− mice. We also show that the predominant antigen recognized by IgA induced upon B. bronchiseptica infection is O-antigen, a protective antigen for some gram-negative bacteria (31, 35). Antibodies against O-antigen also contributed to bacterial clearance from the trachea. Although B. bronchiseptica is very closely related to the human pathogens B. pertussis and B. parapertussis, our data suggest that IgA may not be important to protection against these bacteria. While B. pertussis does not express an O-antigen, B. parapertussis does express this virulence factor and yet does not appear to be substantially affected by the presence or absence of IgA.
MATERIALS AND METHODS
Bacterial strains and growth. B. bronchiseptica strain RB50 (7), B. pertussis strain 536 (33), and B. parapertussis strain 12822 (15) have previously been described. RB50G, Tohama1G, and 12822G are gentamicin-resistant derivatives of these strains and have previously been described (12, 42). B. bronchiseptica strain RB50Δwbm is an isogenic mutant of RB50 that lacks the genes necessary for O-antigen synthesis and has been described previously (32). Bacterial strains were maintained on Bordet-Gengou agar (Difco, Albany, NY) supplemented with 10% defribrinated sheep's blood (Hema Resources, OR) and 20 μg/ml streptomycin or 20 μg/ml gentamicin.
Mouse strains, inoculations, and passive transfers.C57BL/6 mice were obtained from Jackson Laboratories, and IgA−/− mice were a kind gift from Innocent Mbawuike, Baylor College of Medicine, Houston, TX, and have been described elsewhere (13, 27). For inoculations, bacteria were grown in Stainer-Scholte broth with supplements to mid-log phase (optical densities of approximately 0.3 at 600 nm) at 37°C on a roller drum and diluted to approximately 107 CFU/ml. Four- to 6-week-old mice were lightly sedated with 5% isoflurane (IsoFlo; Abbott Laboratories) in oxygen, and 5 × 105 CFU in 50 μl of phosphate-buffered saline were pipetted onto the tip of the external nares. Convalescent mice were generated by inoculating naïve mice and allowing them to recover for at least 28 days. For passive transfer experiments, convalescent mice were bled by cardiac puncture, the serum layer was isolated, and sera were kept at −80°C until use. A 200-μl aliquot of convalescent-phase serum was injected intraperitoneally into naïve mice, immediately followed by inoculation as described above. Mice were maintained in our Bordetella-free breeding facilities at The Pennsylvania State University, and all protocols were reviewed and approved by the university IACUC program.
Harvesting of organs and enumeration of bacteria.For time course experiments, groups of four animals were sacrificed on days 0, 7, 14, 28, 49, 70, or 105 postinoculation and lungs, tracheae, and nasal cavities were collected. Bacterial colonization was determined by homogenization and serial dilution of the indicated organs in phosphate-buffered saline and plating aliquots onto Bordet-Gengou agar with the appropriate antibiotics. Colonies were counted after 2 (for B. bronchiseptica) or 4 (for B. pertussis and B. parapertussis) days of incubation at 37°C.
Rechallenge protocol.For rechallenge experiments, mice were inoculated with 5 × 105 CFU of streptomycin-resistant bacteria as described above. These mice were treated with gentamicin via drinking water (10 mg/ml) for 7 days starting on day 21 postinoculation. On day 28 postinoculation, gentamicin treatment was stopped. On day 30 postinoculation, mice were rechallenged with 5 × 105 CFU of the gentamicin-resistant bacterial strains. These mice were sacrificed at the indicated days post-secondary inoculation for the quantification of both streptomycin-resistant and gentamicin-resistant bacteria.
ELISAs and Western blot assays.Titers of anti-B. bronchiseptica antibodies in convalescent-phase sera were determined by enzyme-linked immunosorbent assay (ELISA) using polyvalent anti-mouse secondary antibodies as described previously (2). Specific classes and isotypes of antibodies were determined by using appropriate secondary antibodies (Southern Biotechnology Associates, Birmingham, AL, and Pharmingen, San Diego, CA). For the analysis of antigens that were recognized by IgA, Westerns blot assays were performed on lysates of B. bronchiseptica strain RB50 and the isogenic mutant strain lacking O-antigen, RB50Δwbm (32). Lysates were run on a 7% sodium dodecyl sulfate-polyacrylamide gel electrophoresis gel under denaturing conditions. Membranes were probed with primary antibodies from pooled B. bronchiseptica-induced convalescent-phase serum at a 1:1,000 dilution or pooled lung homogenate from three wild-type mice infected with B. bronchiseptica 28 days postinoculation at a 1:50 dilution. Detector antibodies were either polyclonal goat anti-mouse (H+L)-horseradish peroxidase conjugated (Southern Biotechnology, Birmingham, AL) or IgA-specific goat anti-mouse IgA-horseradish peroxidase conjugated (Southern Biotechnology, Birmingham, AL). The membrane was visualized with ECL Western blotting detection reagents (Amersham Biosciences, Piscataway, NJ).
Statistics.Three to four mice were used per group for each experiment, and each experiment was performed at least twice with similar results. All quantified values are expressed as the mean ± standard error (error bars). For comparing bacterial numbers or antibody titers between groups, Student's unpaired t tests were used. Differences were assigned statistical significance when the P value was <0.05.
RESULTS
IgA is required for reducing B. bronchiseptica but not B. pertussis or B. parapertussis numbers in the upper respiratory tract.We have previously shown that B-cell-deficient mice fail to clear the bordetellae from the respiratory tract, suggesting that antibodies are critical for bacterial clearance (20). Passive transfer of serum antibodies eliminates B. bronchiseptica, B. pertussis, and B. parapertussis from the lungs and tracheae but not nasal cavities (20, 21, 42) (data not shown), suggesting that local antibodies such as IgA may play an important role in the reduction of bacterial numbers in the upper respiratory tract. To address this possibility, groups of four wild-type and IgA−/− mice were intranasally inoculated with B. bronchiseptica, B. pertussis, or B. parapertussis. Mice were sacrificed on days 0, 7, 14, 28, 49, 70, or 105 postinoculation, and the respiratory organs were collected to quantify the numbers of bacteria in these organs. Numbers of B. bronchiseptica in the lungs of wild-type and IgA−/− mice were similar, indicating that IgA is not essential for reducing the bacterial load in the lungs (Fig. 1A). Approximately 10- to 100-fold-lower numbers of B. bronchiseptica were found in the tracheae of wild-type mice compared to IgA−/− mice at days 14, 28, and 49 postinoculation (Fig. 1B). Furthermore, there were approximately 100-fold fewer bacteria in the nasal cavities of wild-type mice compared to IgA−/− mice from day 14 until the end of our time course (day 105 postinoculation) (Fig. 1C). These results indicate that IgA is important to the reduction of B. bronchiseptica numbers in the trachea and nasal cavity but not the lungs.
B. bronchiseptica colonization of wild type (WT) versus IgA−/− mice. Groups of four C57BL/6 and IgA−/− mice were inoculated with 5 ×105 CFU of B. bronchiseptica. Lungs (A), tracheae (B), and nasal cavities (C) were harvested on days 0, 7, 14, 28, 49, 70, and 105 postinoculation, and the numbers of bacteria were quantified in each organ. Bacterial numbers are expressed as the mean log10 CFU ± the standard error (error bars). The dashed line represents the lower limit of detection.
Interestingly, numbers of B. pertussis were similar in wild-type and IgA−/− mice throughout the respiratory tract (Fig. 2A to C). Although B. parapertussis numbers appeared to be higher in the tracheae and nasal cavities of IgA−/− mice compared to wild-type mice at some time points, there were no significant differences between wild-type and IgA−/− mice throughout the time course (Fig. 2D to F). Together, these data indicate that IgA is not important to clearing B. pertussis or B. parapertussis from the murine host.
B. pertussis and B. parapertussis colonization of wild-type (WT) versus IgA−/− mice. Groups of four C57BL/6 and IgA−/− mice were inoculated with 5 × 105 CFU of B. pertussis (A to C) or B. parapertussis (D to F). Lungs (A and D), tracheae (B and E), and nasal cavities (C and F) were harvested on days 0, 7, 14, 28, 49, 70, and 105 postinoculation, and the numbers of bacteria were quantified. Bacterial numbers are expressed as the mean log10 CFU ± the standard error (error bars). The dashed line represents the lower limit of detection.
IgA is critical to protection against subsequent B. bronchiseptica but not B. pertussis or B. parapertussis infections.In order to control the spread of B. bronchiseptica, sterilizing immunity in the upper respiratory tract, the natural site of colonization by this bacterium, is imperative. We have previously observed that immunity conferred by prior infection is superior to parenteral vaccination in preventing colonization by B. bronchiseptica in a mouse model (12). Infections, but not parenteral vaccines, induce a strong IgA response (12), suggesting that mucosal IgA may render protection conferred by infection-induced immunity more efficiently than that conferred by parenteral vaccines. In order to address this possibility, wild-type and IgA−/− mice were inoculated with B. bronchiseptica as above. Beginning on day 21 postinoculation, mice were treated with gentamicin for 7 days. This treatment completely eliminated bacteria from the lungs and trachea, and only 10 to 100 bacteria remained in the nasal cavities on day 28 postinoculation. These mice were challenged with gentamicin-resistant (GENTr) B. bronchiseptica on day 30 postinoculation. On day 3 postchallenge, the numbers of B. bronchiseptica and GENTrB. bronchiseptica recovered from various respiratory organs were measured. Bacterial numbers from the secondary infection (GENTrB. bronchiseptica) were reduced to levels near the lower limit of detection in the lungs of wild-type and IgA−/− mice (Fig. 3). While numbers were also near the lower limit of detection in the tracheae of wild-type mice, the tracheae of IgA−/− mice harbored ∼104 CFU of GENTrB. bronchiseptica (Fig. 3). Similarly, wild-type mice harbored only hundreds of streptomycin-resistant (STRr) B. bronchiseptica and GENTrB. bronchiseptica in the nasal cavities, whereas IgA−/− mice harbored ∼103 CFU of STRrB. bronchiseptica (data not shown) and ∼106 CFU of GENTrB. bronchiseptica in their nasal cavities (Fig. 3). Similar results were observed on day 7 postchallenge (data not shown). These data indicate that the immune response induced by infection of IgA−/− mice failed to prevent subsequent B. bronchiseptica colonization in the tracheae and nasal cavities.
Colonization of convalescent wild type (WT) and IgA−/− mice by B. bronchiseptica. Groups of four C57BL/6 (WT conv.) and IgA−/− (IgA−/− conv.) mice were inoculated with 5 × 105 CFU STRrB. bronchiseptica. These mice were treated with gentamicin for 7 days beginning on day 21 postinoculation. On day 30, mice were challenged with 5 ×105 CFU of GENTrB. bronchiseptica. Mice were sacrificed on day 3 postchallenge, and respiratory organs were harvested. The numbers of GENTrB. bronchiseptica were quantified in the lungs, tracheae, and nasal cavities and compared to B. bronchiseptica numbers in naïve C57BL/6 mice (WT naïve). Bacterial numbers are expressed as the mean log10 CFU ± the standard error (error bars). The dashed line represents the lower limit of detection.
As individuals vaccinated against whooping cough are able to be infected by B. pertussis and B. parapertussis (6), we sought to test whether IgA is important to the prevention of subsequent infections by these pathogens. Mice were inoculated with B. pertussis or B. parapertussis, then treated with gentamicin as above and, on day 30, challenged with GENTrB. pertussis or GENTrB. parapertussis. In contrast to the results observed with B. bronchiseptica, GENTrB. pertussis numbers were reduced to less than 100 CFU in the lungs, tracheae, and nasal cavities of wild-type and IgA−/− convalescent mice alike (Fig. 4A). Similarly, GENTrB. parapertussis numbers were reduced to less than 100 CFU in the lungs, tracheae, and nasal cavities of wild-type and IgA−/− convalescent mice (Fig. 4B). IgA is induced upon infection by the human-adapted bordetellae (data not shown) (42) but does not appear to be important to preventing subsequent infections by these pathogens in the mouse model.
Colonization of convalescent wild-type (WT) and IgA−/− mice by B. pertussis and B. parapertussis. Groups of four C57BL/6 (WT conv.) and IgA−/− (IgA−/− conv.) mice were inoculated with 5 × 105 CFU STRrB. pertussis or STRrB. parapertussis. These mice were treated with gentamicin for 7 days beginning on day 21 postinoculation. On day 30, mice were challenged with 5 ×105 CFU of GENTrB. pertussis (A) or GENTrB. parapertussis (B), respectively. Mice were sacrificed on day 3 postchallenge, and respiratory organs were harvested. The numbers of gentamicin-resistant bacteria were quantified in the lungs, tracheae, and nasal cavities and compared to those in naïve C57BL/6 mice (WT naïve). The numbers of gentamicin-resistant bacteria are expressed as the mean log10 CFU ± the standard error (error bars). The dashed line represents the lower limit of detection.
IgA−/− mice show normal serum antibody responses to B. bronchiseptica.IgA−/− mice have an altered antibody isotype profile, producing elevated IgG2b and decreased IgG3 in response to influenza viral infection (13). Thus, one possible explanation for the inability of IgA−/− mice to clear B. bronchiseptica infections from the trachea and nasal cavity could be that these mice have an altered serum antibody response to this pathogen. Therefore, we compared the titers of B. bronchiseptica-specific antibodies and isotypes in the sera of wild-type and IgA−/− mice (collected day 28 postinoculation) inoculated with B. bronchiseptica. The overall titers of serum antibodies in wild-type mice were not significantly different from those observed in IgA−/− mice (Fig. 5). Similarly, titers of anti-Bordetella IgM, IgG, IgG1, IgG2a, IgG2b, and IgG3 were indistinguishable between wild-type and IgA−/− mice (Fig. 5). These results suggest that the defect in controlling bacterial numbers in the tracheae and nasal cavities of IgA−/− mice was not due to altered production of other antibody isotypes and support the hypothesis that mucosal IgA is essential to reducing the numbers of B. bronchiseptica in the upper respiratory tract.
B. bronchiseptica-specific antibody titers in sera of wild-type (WT) and IgA−/− mice inoculated with B. bronchiseptica. Groups of C57BL/6 and IgA−/− mice were inoculated with 5 × 105 CFU of B. bronchiseptica. On day 28 postinoculation, sera were collected from these mice and titers of anti-B. bronchiseptica antibodies (Poly) were determined by ELISA along with titers of specific isotypes. Antibody titers are expressed as the mean endpoint titer ± standard error (error bars).
Serum antibody-mediated clearance of B. bronchiseptica from the trachea requires IgA.Previously we showed that passively transferred serum antibodies rapidly clear B. bronchiseptica from the lungs and tracheae of mice (20). Here we sought to determine the contribution of serum IgA to the antibody-mediated clearance of B. bronchiseptica. Groups of four wild-type mice were inoculated with B. bronchiseptica and given a passive transfer of 200 μl of naïve or convalescent-phase serum from wild-type or IgA−/− mice. On day 3 postinoculation, naïve serum-treated mice harbored approximately 106 and 104 CFU of B. bronchiseptica in the lungs and trachea, respectively. Convalescent-phase serum from wild-type mice reduced B. bronchiseptica numbers in the lungs and trachea nearly to the limit of detection by day 3 postinoculation (Fig. 6). IgA-deficient serum reduced B. bronchiseptica to less than 100 CFU in the lungs, but approximately 104 bacteria were still present in the trachea (Fig. 6). As previously observed, passively transferred antibodies had no effect in the nasal cavities of mice (20). Together, these data indicate that IgA is important to serum antibody-mediated clearance of B. bronchiseptica from the trachea.
Effect of convalescent-phase serum from wild-type (WT) versus IgA−/− mice on colonization by B. bronchiseptica. Groups of four C57BL/6 mice were inoculated with 5 × 105 CFU of B. bronchiseptica and immediately given passive transfers of 200 μl of naïve serum (WT NS) or wild-type (WT IS) or IgA−/− (IgA−/− IS) convalescent-phase serum intraperitoneally. On day 3 postinoculation, respiratory organs were harvested and bacterial numbers were quantified in the lungs, tracheae, and nasal cavities. Bacterial numbers are expressed as the mean log10 CFU ± the standard error (error bars). The dashed line represents the lower limit of detection.
The IgA response is primarily directed against the O-antigen of B. bronchiseptica.Since IgA was found to be necessary for efficient reduction of bacterial numbers in the upper respiratory tract, we sought to determine the bacterial factors that were being recognized by IgA. Western blots with lysates of wild-type B. bronchiseptica or an isogenic mutant lacking O-antigen were probed with serum or lung homogenate from convalescent wild-type mice. Horseradish peroxidase-conjugated antibodies specific for all murine antibody isotypes (polyclonal) or IgA only were used to identify the antigens that were bound by IgA. We found that IgA in the serum and lungs primarily recognized a low-molecular-weight smear that was present in the wild-type B. bronchiseptica strain but absent from the strain lacking O-antigen (Fig. 7). This indicated that IgA induced by B. bronchiseptica infection primarily recognizes the O-antigen of that bacterium.
Recognition of B. bronchiseptica antigens by antibodies in convalescent-phase serum or lung homogenate. Groups of three C57BL/6 mice were inoculated with 5 × 105 CFU of B. bronchiseptica. Twenty-eight days later, sera and lungs were collected. Western blot assays were performed using lysates from wild-type B. bronchiseptica (Bb) or the O-antigen-deficient strain (BbΔwbm) probed with pooled convalescent-phase serum or lung homogenate. Horseradish peroxidase-conjugated secondary antibodies specific for all murine antibody isotypes (polyclonal) or only IgA were used for detection.
O-antigen-specific antibodies are important to clearance of B. bronchiseptica from the trachea.The fact that IgA was protective in the trachea and recognized O-antigen led us to speculate that antibodies against O-antigen are important to bacterial clearance from the trachea. Serum raised against the O-antigen-deficient B. bronchiseptica strain did not contain antibodies to all antigens that were recognized by serum from mice infected with the wild-type strain but did contain antibodies to antigens that are known to be protective antigens of B. bronchiseptica (data not shown) (24). The largest difference between the sera was the absence of O-antigen-specific antibodies in the serum raised against the O-antigen-deficient strain. To test if O-antigen is an important protective antigen, groups of four wild-type mice were inoculated with B. bronchiseptica and given a passive transfer of serum raised against wild-type B. bronchiseptica or an isogenic B. bronchiseptica strain lacking O-antigen. Mice were then sacrificed 3 days later to quantify bacterial numbers throughout the respiratory tract. Mice given passive transfers of serum raised against wild-type or O-antigen-deficient B. bronchiseptica were able to reduce bacterial numbers in the lungs to less than 100 CFU within 3 days. Serum raised against wild-type bacteria reduced B. bronchiseptica numbers in the trachea, but serum raised against the O-antigen-deficient strain was ineffective against B. bronchiseptica in the trachea (Fig. 8). Along with the facts that serum IgA is required for clearance of B. bronchiseptica from the trachea and primarily recognizes O-antigen, these data suggest that O-antigen-specific IgA mediates bacterial clearance from the trachea. Interestingly, serum raised against the O-antigen-deficient strain appeared to have a greater effect on bacterial numbers in the nasal cavity than serum raised against wild-type bacteria (Fig. 8).
Effects of antibodies against O-antigen on colonization by B. bronchiseptica. Groups of four C57BL/6 mice were inoculated with 5 × 105 CFU of B. bronchiseptica and immediately given passive transfers of 200 μl of naïve serum (NS), B. bronchiseptica convalescent-phase serum (Bb IS), or B. bronchisepticaΔwbm convalescent-phase serum (BbΔwbm IS). On day 3 postinoculation, respiratory organs were harvested and bacterial numbers were quantified in the lungs, tracheae, and nasal cavities. Bacterial numbers are expressed as the mean log10 CFU ± the standard error (error bars). The dashed line represents the lower limit of detection.
DISCUSSION
Although we have a relatively detailed understanding of the mechanisms by which B. bronchiseptica, B. pertussis, and B. parapertussis are eliminated from the lower respiratory tract (20, 21, 42), less is known about how these pathogens are controlled in the nasal cavity. Here we show that IgA plays an important role in controlling B. bronchiseptica numbers in the upper respiratory tract. IgA induced upon infection by B. bronchiseptica recognized a limited number of antigens and was largely specific for the O-antigen of this bacterium, and both IgA and O-antigen-specific antibodies were important to eliminating B. bronchiseptica from the trachea. In contrast, IgA did not appear to have a protective role against B. pertussis or B. parapertussis. While IgA may not be important to protecting against the human-adapted bordetellae, it is also possible that this is an artifact of the mouse model, as we will discuss below.
IgA can mediate protection against bacterial and viral pathogens through a number of mechanisms. Secretory IgA is a part of the first line of defense, neutralizing and agglutinating pathogens at the mucosal surface (30, 38). Serum IgA may act as a second line of defense, acting against pathogens that have breached the mucosal surface (30, 38). Both forms of IgA modulate local inflammation, although secretory IgA appears to induce anti-inflammatory signals and serum IgA induces proinflammatory signals (3, 38). Serum IgA also facilitates antibody-dependent cell-mediated cytotoxicity and Fcα-mediated phagocytosis (29, 40). Our data suggest that serum IgA, although there is little IgA in the serum of B. bronchiseptica-convalescent mice, is necessary for protection against B. bronchiseptica in the trachea, since a passive transfer of serum antibodies from wild-type mice led to a reduction of bacterial numbers but serum from IgA−/− mice did not (Fig. 6). The fact that a similar trend was not observed in the nasal cavity upon the passive transfer of antibodies could have been the result of inefficient transport of serum antibodies to the nasal epithelium. Alternatively, secretory IgA may be more important to protection in the nasal cavity while serum IgA may mediate bacterial clearance from the trachea. That being said, the specific roles of secretory and serum IgA upon B. bronchiseptica remain unclear.
Mucosal bacterial pathogens, such as Streptococcus, Neisseria, and Bordetella species, continue to circulate despite the widespread use of vaccines (5, 9, 14, 28, 37, 41). This may, in part, be attributed to current vaccine strategies which are aimed at inducing serum antibodies to specific virulence factors and therefore protect against severe disease but do not prevent infection. We and others have observed that serum antibodies are ineffective against some mucosal pathogens in the upper respiratory tract (20, 34). It is therefore important for vaccines to elicit appropriate local immune responses that can prevent colonization by pathogens if there is a desire to not only protect from disease but also prevent infection. Because of this, some recent immunization strategies have emphasized the induction of a strong, local IgA response using mucosal vaccines to try to curb the spread of infectious diseases.
Care must be taken when extrapolating studies in a mouse model to human models of infection and disease. There are substantial differences in IgA-mediated immunity between mice and humans regarding the IgA receptors that they express (8). Although IgA was not required to clear B. pertussis and B. parapertussis in the mouse model, this isotype may be important in humans. For example, B. parapertussis appears to persist for life in the nasal cavity in the murine model (20, 42), but this does not appear to be the case in humans. Thus, clearance of B. parapertussis from the human nasal cavity may be dependent on some IgA-dependent mechanism that is lacking in mice. Additionally, Hellwig et al. showed that human Fcα receptor I-transgenic mice are able to more efficiently control B. pertussis than nontransgenic mice (16), suggesting that IgA-mediated immunity to B. pertussis may be more important in human hosts. Thus, whooping cough vaccines that induce Bordetella-specific IgA could prove to be more efficacious than current strategies (10).
Unlike the human-adapted bordetellae, B. bronchiseptica naturally colonizes and chronically persists in the nasal cavity and trachea of a variety of mammals (11), and IgA mediates protection in these respiratory compartments. Poor induction of B. bronchiseptica-specific IgA could partially explain the lack of efficacy of current B. bronchiseptica vaccines. Some current vaccine strategies involve the use of live, attenuated B. bronchiseptica strains (26), and we have recently shown that infection by a genetically defined avirulent B. bronchiseptica strain, AVS, confers efficient immunity to B. bronchiseptica (24). Utilizing vaccine strategies consisting of intranasal inoculations with live, attenuated strains that induce strong, B. bronchiseptica-specific IgA responses may aid in preventing the spread of this pathogen among domestic animal populations.
ACKNOWLEDGMENTS
This study was supported by NIH grant AI 053075 (E.T.H.).
We have no conflicting financial interests.
We thank Innocent Mbawuike for the generous donation of IgA−/− mice.
FOOTNOTES
- Received 20 March 2007.
- Returned for modification 19 April 2007.
- Accepted 13 June 2007.
- Copyright © 2007 American Society for Microbiology
