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Infection and Immunity, October 2005, p. 6508-6513, Vol. 73, No. 10
0019-9567/05/$08.00+0 doi:10.1128/IAI.73.10.6508-6513.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Department of Veterinary Science, The Pennsylvania State University, University Park, Pennsylvania 16802
Received 9 May 2005/ Returned for modification 13 June 2005/ Accepted 1 July 2005
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B. pertussis and B. parapertussis appear to have diverged independently from a B. bronchiseptica-like progenitor, with the latter emerging more recently. Both species seem to have lost a number of genes during the adaptation to infect a human host but still share many of their known virulence factors (11, 29). Despite the similarities between the two species, epidemiological and vaccine studies have shown that B. parapertussis is commonly found in populations vaccinated against B. pertussis and that B. pertussis vaccination may have little, if any, effect on B. parapertussis infection and disease (2, 16, 24, 40). However, experimental studies in a murine model have suggested that infection-induced immunity may induce stronger cross-protection (38). B. pertussis has received the vast majority of attention from the scientific community, since it has been historically associated with whooping cough, but the recent evidence of increases in the prevalence of B. parapertussis have spurred increased interest. Therefore, the relevant immune functions that provide protective immunity to B. parapertussis may be of increasing importance.
While it has previously been shown that infection by B. parapertussis or B. pertussis induces an immune response to B. parapertussis, that response has largely gone uncharacterized (38, 39). To understand the mechanisms of protective immunity to B. parapertussis, we dissected the host immune functions necessary for elimination of the bacterium from the LRT. We have recently shown that in a mouse model of infection, clearance of B. pertussis from the LRT requires antibodies (Abs), T cells, Fc
receptors, and neutrophils (polymorphonuclear leukocytes [PMNs]) (23; unpublished data). Here we show that clearance of B. parapertussis from the LRT similarly requires Abs, T cells, and PMNs. However, unlike immunity to B. pertussis, Fc
receptors are not essential, while the complement cascade is required. Serum Abs and helper T cells together are sufficient to eliminate B. parapertussis from the LRT, while mucosal Abs and cytotoxic T lymphocytes are not required. These data suggest that mechanisms of protective immunity to B. parapertussis and B. pertussis are not identical.
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Animal experiments.
C57BL/6, RAG2/, µMT, TCR
/, C5/, and Cd11b/ mice were obtained from Jackson Laboratory (Bar Harbor, Maine). FcR2/
-common/ mice were obtained from Taconic Laboratories (Germantown, New York). C3/ mice, backcrossed extensively onto a C57BL/6 background, have been described elsewhere and were a gift of Rick Wetsel (5). IgA/ mice were a kind gift from Innocent Mbawuike (26). All mice were bred in a Bordetella-free environment. Mice were slightly sedated with isofluorane (Abbott Laboratories) and inoculated by pipetting 50 µl of phosphate-buffered saline (PBS) containing approximately 5 x 105 B. parapertussis onto the tip of the external nares as previously described (22). For time course experiments, groups of three or four animals were sacrificed on days 3, 7, 14, 28, 49, 70, and/or 105 postinoculation. Lung and systemic colonization was quantified by homogenizing tissues in PBS, plating onto Bordet-Gengou blood agar containing 20 µg/ml gentamicin, and counting the number of colonies. Statistical analysis was performed using the Student t test when comparing numbers of CFU. Values yielding P values less than 0.05 were considered statistically significant. Adoptive transfer experiments were conducted by injecting 200 µl of convalescent-phase serum intraperitoneally (i.p.) at the time of inoculation. Convalescent-phase serum samples were collected from wild-type mice infected with B. parapertussis 28 days postinoculation. For the production of survival curves, once the progression of disease was clear, moribund animals were euthanized to prevent unnecessary stress. All animals were handled in accordance with institutional guidelines.
Cell depletions. PMN depletion was done by injecting 1 mg of the monoclonal Ab from the RB6-8C5 hybridoma i.p. (8, 9) 24 hours prior to and 7 days after infection. CD4+ T cells were depleted by injections of 1 mg of the monoclonal Ab from the GK1.5 hybridoma at days 0 and 7. Similarly, CD8+ T cells were depleted by injection of 1 mg of the monoclonal Ab from the YTS168 · 4 hybridoma at days 0 and 7 (31).
Enzyme-linked immunosorbent assays (ELISAs). Bacteria were grown overnight to an optical density at 600 nm of 0.7, heat inactivated, diluted in carbonate buffer, and used to coat the wells on 96-well plates. Plates were stored at 4°C (wells filled with PBS containing Tween 20 [PBS-T] plus 1% bovine serum albumin) until use. A 1:50 dilution of convalescent-phase serum samples from different mouse strains was added to the first wells and serially diluted 1:2 across the plates. The plates were incubated for 2 h at 37°C in a humidified chamber and then washed three times with PBS-T. Polyvalent anti-mouse secondary Abs were used to look at the total titer (1), and specific isotypes were determined by using the appropriate secondary Abs (Southern Biotechnology Associates and Pharmingen). The plates were then incubated at 37°C in a humidified chamber for 1 h before they were washed four times with PBS-T. 2,2'-Azino-bis(3-ethylbenz-thiazoline-6-sulfonic acid) in a phospho-citrate buffer and hydrogen peroxide were added to the wells, which were then incubated at room temperature in the dark for 30 min. A sodium fluoride solution was added to the wells to stop the reaction, and the plates were read at an absorbance of 405 nm.
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/ (T
ß-cell-deficient) mice were given an intranasal inoculation of 5 x 105 CFU of B. parapertussis in 50 µl PBS. Wild-type, µMT, and TCR-
/ mice survived 105 days postinoculation without any signs of distress and were then euthanized. On day 17 postinoculation, RAG2/ mice began to show symptoms of disease, such as hunched posture, decreased level of activity, and ruffled fur. These mice began dying on day 18, and all were dead by day 28 (Fig. 1A). In a subsequent experiment, B. parapertussis numbers in the LRT and blood were measured for the same mouse strains on day 28 postinoculation. Wild-type C57BL/6 mice reduced B. parapertussis to about 100 CFU in the LRT by day 28. However, RAG2/, µMT, and TCR-
/ mice were all unable to reduce bacterial numbers below the maximal levels observed on day 7 in wild-type mice (
106 to 107 CFU). Although each of these immunodeficient mouse strains was unable to efficiently reduce colonization in the lungs (Fig. 1B), Rag2/ mice were the only strain that allowed colonization of the blood by B. parapertussis (Fig. 1C). The observation that only mice lacking both B cells and T cells succumb to lethal, systemic B. parapertussis infection suggests that either B cells or T cells can control the spread from the respiratory tract.
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FIG. 1. Adaptive immunity is required for prevention of systemic spread and death due to B. parapertussis infection. Groups of 10 C57BL/6 (C57) ( ), 10 RAG2/ (X), 10 µMT ( ), and 10 TCR- / ( ) mice were intranasally inoculated with 5 x 105 CFU of B. parapertussis in 50 µl of PBS for a survival curve (A). Separate groups of four similarly inoculated mice of each strain were sacrificed at day 28 to measure colonization of the lungs (B) and blood (C). The number of bacteria recovered from each tissue is expressed as the log10 mean ± standard deviation (error bar). Values that were significantly different from those for wild-type C57BL/6 mice (P values of less than 0.05) are indicated by asterisks. The dashed line represents the limit of detection.
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/ mice for 105 days to determine whether they were able to clear the infection. Groups of 24 mice were infected, and 4 mice of each strain were sacrificed on days 7, 14, 28, 49, 70, and 105. C57BL/6 mice reduced B. parapertussis to low numbers in the LRT by day 28, and no bacteria could be recovered by day 49 postinoculation. RAG2/, µMT, and TCR-
/ mice were all defective in bacterial clearance compared to wild-type mice, and high numbers of B. parapertussis were still present in the LRT at 105 days postinoculation (Fig. 2A). Serum samples were taken from mice sacrificed on day 28 postinoculation to quantify the titers of anti-B. parapertussis polyclonal Abs, immunoglobulin A (IgA), IgG, IgG1, IgG2a, IgG2b, IgG3, and IgM by ELISAs. Anti-B. parapertussis IgA and IgG3 were undetectable in the sera of any infected mice (data not shown). While C57BL/6 mice produced high titers of anti-B. parapertussis Abs of other isotypes, µMT and TCR-
/ mice were defective in the production of all isotypes except for the production of IgM by TCR-
/ mice (Fig. 2B). Mouse strains that were unable to produce anti-B. parapertussis Abs were also defective in the clearance of the bacteria from the LRT, suggesting that Abs may be critical to the elimination of B. parapertussis from the LRT.
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FIG. 2. Mice deficient in Ab production are unable to clear B. parapertussis from the LRT. Groups of 24 C57BL/6 ( ), RAG2/ (X), µMT ( ), and TCR- / ( ) mice were inoculated as described in the legend to Fig. 1 and sacrificed on days 7, 14, 28, 49, 70, and 105 (A). The numbers of bacteria are expressed as the log10 means ± standard deviations (error bars). Pooled serum samples were taken from C57BL/6 (black bars), µMT (white bars), and TCR- / (hatched bars) mice sacrificed on day 28 for quantification of anti-B. parapertussis (anti-Bpp) antibodies via an ELISA (B). Dashed lines represent the limit of detection.
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FIG. 3. Mucosal antibodies are not crucial to clearance of B. parapertussis from the LRT. Groups of 24 C57BL6 ( ) and IgA/ ( ) mice were inoculated as described in the legend to Fig. 1 and sacrificed on days 7, 14, 28, 49, 70, and 105. The numbers of bacteria are expressed as log10 means ± standard deviations (error bars). The dashed line represents the limit of detection.
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105 CFU) (Fig. 4). In contrast, both C57BL/6 and µMT mice treated with immune serum lowered bacterial numbers to less than 100 CFU in the LRT by day 14, indicating that Abs are very effective in these mice. However, treatment with immune serum had no effect on the numbers of B. parapertussis in RAG2/ or TCR-
/ mice (Fig. 4), indicating that
ß T cells are required for the function of Abs. To determine whether cytotoxic T lymphocytes and/or helper T cells are necessary for Ab-mediated clearance of B. parapertussis, two more groups of C57BL/6 mice inoculated with B. parapertussis and treated with immune serum were also injected i.p. with anti-CD4 or anti-CD8 Abs at days 0 and 7. Mice depleted of CD8+ T cells were able to clear the infection from the LRT by day 14, but those lacking CD4+ T cells were unable to do so (Fig. 4). These data suggest that not only are T cells required for the production of anti-B. parapertussis Abs but that CD4+ cells are also involved in their function.
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FIG. 4. T cells are required for the function of serum antibodies in clearing B. parapertussis from the LRT. Four control C57BL/6 (C57) mice were given an i.p. injection of naïve serum (NS) at the time of intranasal inoculation with B. parapertussis as described in the legend to Fig. 1. Groups of four C57BL/6, RAG2/, µMT, and TCR- / mice were given i.p. injections of immune serum (IS) at the time of inoculation. Groups of four C57BL/6 mice were given i.p. injections of anti-CD4 ( CD4) or anti-CD8 ( CD8) antibody in addition to immune serum at the time of inoculation. All mice were sacrificed on day 14. The numbers of bacteria are expressed as the log10 means ± standard deviations (error bars). Values that were significantly different from those for wild-type C57BL/6 mice (P values of less than 0.05) are indicated by asterisks. The dashed line represents the limit of detection.
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receptors are not.
Several host immune factors were analyzed to determine the mechanism by which Abs facilitate the elimination of B. parapertussis from the LRT. Aspects of the complement cascade were examined using C3/ and C5/ mice. In mice lacking C3, and therefore the entire complement cascade, Abs were unable to clear the bacteria by day 14. However, in mice lacking only C5, Abs greatly reduced B. parapertussis numbers (approximately 99% reduction), although not to the limit of detection (Fig. 5). The roles of CR3 and Fc
receptors, which bind opsonized C3b and antibodies, respectively, were also analyzed using mice lacking CD11b (CD11b/) and mice lacking all three Fc
receptors (Fc
R2/,
common/). Both mouse strains were able to significantly decrease numbers of B. parapertussis by day 14 postinoculation (approximately 99% clearance), although they did not completely clear the infection (Fig. 5). These results suggest that Fc
receptors are not required for efficient reduction of B. parapertussis numbers, although they may contribute to clearance. Together these data indicate that only complement factors upstream of C5 activation are required for the efficient function of serum Abs against B. parapertussis.
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FIG. 5. Complement is required for the function of serum antibodies against B. parapertussis. Four control C57BL/6 (C57) mice were given i.p. injections of naïve serum (NS) upon inoculation with B. parapertussis as described in the legend to Fig. 1. Groups of four C57BL/6, C3/, C5/, CD11b/, and FcR- / mice were injected i.p. with immune serum (IS) at the time of inoculation. All mice were sacrificed on day 14. The numbers of bacteria are expressed as the log10 means ± standard deviations (error bars). Values that were significantly different from those for wild-type C57BL/6 mice (P values of less than 0.05) are indicated by asterisks. The dashed line represents the limit of detection.
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FIG. 6. Neutrophils are required for the function of serum antibodies against B. parapertussis. Four control µMT mice were injected i.p. with naïve serum (NS) upon inoculation as described in the legend to Fig. 1. Four µMT mice were injected i.p. with immune serum (IS) upon inoculation. Four µMT mice were injected with the monoclonal antibody RB6-8C5 (RB6) to deplete neutrophils 24 h prior to and 7 days after infection and injected with immune serum upon inoculation. All mice were sacrificed on day 14. The numbers of bacteria are expressed as the log10 means ± standard deviations (error bars). Values that were significantly different from those for wild-type C57BL/6 mice (P values of less than 0.05) are indicated by asterisks. The dashed line represents the limit of detection.
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receptors and/or CR3. In this model, either Fc
receptors or CR3 are sufficient for reduction of the bacterial load (about 99% clearance), but the presence of both receptors results in complete elimination of bacteria. Furthermore, our data also suggest that helper T cells, but not cytotoxic T cells, are necessary for the function of Abs. Although the exact role of helper T cells in facilitating the bacterial clearance is not known, our preliminary studies suggest that they aid in the recruitment and perhaps activation of PMNs in the lungs via gamma interferon production (unpublished data). It appears that both humoral and cellular responses are crucial in immunity to B. parapertussis. Recent clinical surveys indicate that B. parapertussis may be responsible for a large percentage of diagnosed cases of whooping cough (40). Although the immune response to B. pertussis is fairly well characterized, the immune response to B. parapertussis has largely been ignored. Our data show that like B. pertussis, both humoral and cellular immune responses are required for sterilizing immunity to B. parapertussis. Since these two species are closely related, it would be expected that protective immunity induced by either one would induce cross-protection against the other. In fact, experimental studies have shown that these two species induce a limited level of cross-protection, clearly not as high as their levels of self-protection (10, 18, 21, 41). This incomplete cross-protection could result from the two species being antigenically distinct in each of their many surface and secreted proteins. Alternatively, a different set of prominent surface molecules could also prevent effective immune cross-protection. O antigen is the dominant surface antigen of B. parapertussis but is absent from B. pertussis isolates. Our previous work has shown that O antigen is required for efficient colonization of the murine respiratory tract by B. parapertussis (3), but its role in eliciting a protective immune response or conversely, protecting bacteria from the adaptive immune response is largely unknown. O antigen is expressed by several different bacteria and has various functions in vivo and in vitro, including contributing to host colonization, resistance to complement-mediated killing, and the ability to cause sepsis (3, 6, 27, 34, 35). Our previous studies have shown that the O antigens of B. parapertussis and B. bronchiseptica protects them from killing by complement in the absence of Abs, while B. pertussis, which lacks this virulence factor, is naturally sensitive to the effects of naïve serum complement (3). It is unknown at this time whether O antigen has other functions in B. parapertussis infections in addition to conferring resistance to complement-mediated killing. As a major component of the outer surface of the bacterium, it is possible that it either shields other protective antigens or presents a decoy antigen. The lack of expression of pertussis toxin (ptx) by B. parapertussis is another key difference between this bacterium and B. pertussis. Compared to wild-type B. pertussis strains, strains deleted of ptx are not able to colonize the respiratory tract as efficiently (4). However, B. parapertussis is able to reach high levels in the LRT without this toxin. ptx is a prominent antigen of B. pertussis to which high titers of specific Abs are produced (38). Thus, it is possible that by not expressing this toxin, B. parapertussis may avoid B. pertussis-induced immunity.
Ecological theory holds that two antigenically similar pathogens cannot occupy the same host population indefinitely due to immune-system-mediated competition; the more virulent of the two will have the short-term advantage of increased transmission but is more likely to undergo epidemic fadeout (13). The coexistence of B. parapertussis and B. pertussis in the same host population, and occasionally in the same individual (16), indicates that these organisms have some mechanism to avoid immune-system-mediated competition. Phylogenetic analysis suggests that B. parapertussis emerged from a B. bronchiseptica-like progenitor and adapted to humans more recently than B. pertussis did (29). Thus, it could be predicted that B. parapertussis successfully invaded a human population in which B. pertussis was already prevalent and, consequently, needed to develop mechanisms to avoid B. pertussis-induced immune responses. The presence of B. parapertussis and B. pertussis in the same population is intriguing, and understanding their abilities to coexist could provide an excellent model to study evolution, adaptation, and spread in the emergence of bacterial pathogens.
While whooping cough has recently been increasing in prevalence in vaccinated populations, the dangers of B. parapertussis and B. pertussis infections lie in the spread of these bacteria to unvaccinated individuals or those in whom immunity has waned. Both species are quite capable of colonizing B. pertussis-vaccinated individuals, often going undiagnosed (2, 16, 40), and whooping cough epidemics are frequent and periodic in some populations (15, 33). Control of these pathogens at the population level requires greater understanding of immune mechanisms that could provide protection not only against severe disease but also against subclinical infections. Such understanding would allow for the design of novel vaccine strategies that induce robust and long-lasting humoral and cellular immune responses against both B. parapertussis and B. pertussis.
We thank all members of the Harvill lab for critical reading of the manuscript and scientific discussions. We also thank Rick Wetsel for the donation of C3/ mice, Innocent Mbawuike for the donation of IgA/ mice, and Gary Huffnagle for the donation of hybridomas.
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