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Infection and Immunity, December 2002, p. 7120-7125, Vol. 70, No. 12
0019-9567/02/$04.00+0     DOI: 10.1128/IAI.70.12.7120-7125.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Construction of an Actinobacillus pleuropneumoniae Serotype 2 Prototype Live Negative-Marker Vaccine

Walaiporn Tonpitak,1,{dagger} Nina Baltes,1 Isabel Hennig-Pauka,2 and Gerald F. Gerlach1*

Institut für Mikrobiologie und Tierseuchen,1 Klinik für Kleine Klauentiere, Tieraerztliche Hochschule Hannover, 30173 Hanover, Germany2

Received 11 June 2002/ Returned for modification 22 July 2002/ Accepted 4 September 2002


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ABSTRACT
 
Deletions were introduced into the ureC and apxIIA genes of an Actinobacillus pleuropneumoniae serotype 2 strain by homologous recombination and counterselection. The double-mutant contains no foreign DNA, is highly attenuated, protects pigs from homologous challenge upon a single aerosol application, and facilitates the serological discrimination of immunized and infected herds.


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TEXT
 
Vaccination against bacterial pathogens is increasingly relevant in the livestock industry because of the growing problems of drug resistance and antimicrobial residues (22). Frequently, vaccination is directed against pathogens already present in the herd (3). This metaphylactic strategy can decrease the frequency of clinical disease and thereby significantly lower the need for antibiotic therapy. However, bacterial vaccines currently in use do not allow a serology-based discrimination between vaccinated and infected animals in a routine test. This discrimination, however, is of major importance for generating and maintaining specified pathogen-free herds which are the optimum choice with respect to long-term animal health and consumer protection (11).

Actinobacillus pleuropneumoniae, the cause of porcine pleuropneumonia, is a major economic problem in the swine industry worldwide (7). Treatment of the disease, characterized by hemorrhagic, fibrinous, and necrotic lesions, is increasingly difficult due to the occurrence of antibiotic-resistant strains (4). Vaccination is complicated by limited cross-serotype protection upon immunization with bacterin preparations (14). In addition, bacterin vaccines do not prevent colonization, thereby potentially facilitating the development of healthy carrier animals; in contrast, convalescent pigs are completely protected against infection with a homologous serotype (5).

Here we describe the development of a prototype live attenuated A. pleuropneumoniae serotype 2-negative marker vaccine carrying deletions in the apxIIA and ureC genes. We investigate whether this strain is attenuated and whether it can prevent not only clinical disease but also colonization upon a single application. An A. pleuropneumoniae serotype 2 strain was chosen since it is the most frequently isolated serotype in northern Europe. The apxIIA gene was deleted as it encodes a highly immunogenic virulence factor expressed by all A. pleuropneumoniae serotypes except serotype 10; it has been used for serodiagnosis (16) and, therefore, could be used for discrimination of immunized and infected herds in routine diagnostics. The ureC gene was deleted in order to potentially reduce shedding of the vaccine strain (2); in additon, it can serve as a reliable phenotypic marker to discriminate between the vaccine and the wild-type strain.

Construction of a mutant strain. To construct the A. pleuropneumoniae serotype 2 isogenic mutant, 12 clinical isolates were tested initially with respect to their amenability to genetic manipulation via conjugation and cointegration of pBMKU{Delta}1 (Table 1). One isolate, designated A. pleuropneumoniae C5934, formed stable cointegrates upon conjugation (19) as assessed by DNA colony blots (21) and was used for further manipulations. For sucrose counterselection, which is required to obtain unmarked deletion mutants, a single kanamycin-resistant colony was cultured in 1 ml of supplemented PPLO medium (Difco, Detroit, Mich.) at 37°C for 2 h with shaking. Then, an equal volume of counterselection medium (0.4 volumes of 2x medium without added NaCl [46 g of Bacto Beef Heart for Infusion/liter, heated and filtered as recommended by the manufacturer, plus 9.25 g of Bacto Peptone/liter, both purchased from Difco], 0.5 volume of 40% sucrose, 0.1 volume of equine serum) was added, and the incubation was continued for 6 h. Ten sterile glass beads (2 mm in diameter) were added, and bacterial clumps were broken by vortexing for 2 min. Aliquots were plated and further investigated by PCR analyses (1) with the appropriate primers (Table 1); the PCR consisted of an initial denaturation (94°C, 30 s), 32 amplification cycles (denaturation [94°C, 30 s], annealing [53°C, 40 s], and extension [72°C, 2 min]), and a final extension (72°C, 10 min). Colonies with the correct PCR profile (Fig. 1A) were confirmed by Southern blot analyses upon capillary transfer (21) with the PCR products obtained from the respective deletion mutants as a probe (Fig. 1B). The absence of gross chromosomal rearrangements was shown by pulsed-field gel electrophoresis (Fig. 1C) performed as previously described (18). The resulting A. pleuropneumoniae double mutant was urease negative and showed a CAMP-like hemolytic activity on Columbia sheep blood (CSB) agar plates with S. aureus (Fig. 1D). These results show that some isolates of A. pleuropneumoniae serotype 2 are amenable to genetic manipulation by a conjugation system previously described for A. pleuropneumoniae serotype 7.


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  		TABLE 1. Characteristics of bacterial strains, plasmids, and primers used in this study



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  		FIG. 1. Analysis of A. pleuropneumoniae C5934wt (lanes 1) and A. pleuropneumoniae C5934{Delta}ureC{Delta}apxIIA (lanes 2). (A) PCR with the primers ureC2 and ureX (left) and apxIIAU and apxIIAL (right). The reduction of the size of the PCR fragments obtained from the double mutant is due to the deletion of an 870-bp SphI-BstEII fragment in the ureC gene (19) and a 1,518-bp BglII-NcoI fragment in the apxIIA gene (Table 1). (B) Southern blot analyses by using the truncated ureC gene (left) and the truncated apxIIA gene (right) as probes; chromosomal DNA was digested with BstEII (ureC blot) and NdeI (apxIIA blot); the reduction of the size in the hybridizing restriction endonuclease fragments obtained in the mutants is as calculated based on the size of the deleted fragments. (C) Pulsed-field gel electrophoresis of ApaI-, AscI-, and NotI-digested DNA showing that no gross rearrangements have occurred. (D) Hemolysis and CAMP-like phenomenon of the A. pleuropneumoniae double mutant due to a Staphylococcus aureus strain on CSB agar.

Virulence studies. The degree of attenuation of the A. pleuropneumoniae double mutant was investigated by using three groups of eight pigs each as described previously for A. pleuropneumoniae serotype 7 (2). The results showed a significant reduction in clinical symptoms and pathology even with the higher dose (Fig. 2A). The bacteriological examination at the end of the experiment revealed that A. pleuropneumoniae could be consistently reisolated in high numbers from the lung lesions of pigs challenged with the wild-type strain (Fig. 2A). In pigs challenged with the double mutant, A. pleuropneumoniae could be reisolated from intact lung tissue in small numbers in the low-dose challenge group and from lesions in the high-dose challenge group (Fig. 2A). In all pigs, an immune response could be detected in the detergent extract enzyme-linked immunosorbent assay (ELISA) (10), and only in the wild-type group six of seven pigs showed elevated levels (>10 ELISA units [EU]) in the ApxIIA ELISA (16) 3 weeks after challenge (Fig. 2B). These results showed that the A. pleuropneumoniae double-mutant strain is highly attenuated and that the group infected with the mutant strain can be discriminated from the wild-type-infected group based on the ApxIIA ELISA; the antigenic relationship among A. pleuropneumoniae Apx toxins described previously (17) did not cause any false-positive reaction in the recombinant ApxII-ELISA used.



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  		FIG. 2. Lung lesion score (A) and humoral immune response (B) at 21 days after challenge with the A. pleuropneumoniae parent strain and isogenic double mutant. Pigs were aerosol infected with a total of 1.3 x 105 A. pleuropneumoniae C5934wt and a total of 1.3 x 105 or 6.5 x 105 A. pleuropneumoniae C5934{Delta}ureC{Delta}apxIIA. The aerosolization of a total of 105 bacteria results in 102 A. pleuropneumoniae per liter of aerosol in the chamber. Lung lesion scores were determined as described by Hannan et al. (12), and the bacteriological examination was done as previously described (2). Mortality (Mo) and reisolation frequency from lung tissue (RI) results for the different groups are given. The antibody response was assessed with two ELISAs with a detergent extract (solid symbols) and the recombinant ApxIIA protein (open symbols) as the solid-phase antigen. The response was quantified as the serum titer in comparison to an internal negative control (detergent extract ELISA) and in EU (based on an external standard) in the ApxIIA ELISA. For the standardized ApxIIA ELISA, activities of <=10 EU in the sera are considered negative, 11 to 25 EU is intermediate, and >25 EU is a positive result. The central square within the hourglass shape represents the geometric mean, the hinges present the values in the middle of each half of data, and the top and bottom squares mark the maximum and minimum values, respectively. The asterisk denotes statistical significance (P < 0.05) for the lung lesion score (Wilcoxon test).

Protection studies. Protection experiments were performed by using the A. pleuropneumoniae serotype 2 double mutant (A. pleuropneumoniae C5934{Delta}ureC{Delta}apxIIA) as a live vaccine in a single aerosol immunization. Three weeks later the immunized group and a nonimmunized control group were challenged with the virulent parent strain. No clinical symptoms were observed in immunized pigs, whereas six of seven pigs in the control group developed severe disease with two pigs dying on the second day after challenge. Postmortem analysis on day 7 postinfection revealed no pathological changes in six of seven immunized pigs (Fig. 3A). The pathological changes in the remaining pig were due to the vaccine strain since only this strain could be isolated; the vaccine strain could not be isolated from any of the other pigs. The challenge strain (A. pleuropneumoniae C5934wt) was isolated in low numbers from unaltered lung tissue of one of the seven pigs. In the control group, confluent growth of the challenge strain was obtained from the lesions of six of seven pigs (Fig. 3A). None of the pigs in either group showed a detectable immune response in the ApxIIA ELISA before challenge. The lack of a detectable response in the wild-type group 1 week after challenge was to be expected, since a minimum of 10 days is required to obtain a detectable serum activity (data not shown). In the detergent extract ELISA, all vaccinated pigs and none of the controls responded before challenge (Fig. 3B). At 1 week after challenge, the specific titer in the sera of immunized pigs had increased slightly, and also all nonimmunized pigs had developed very low titers in the detergent extract ELISA (Fig. 3B). The results show that a single aerosol application of the attenuated double mutant resulted in protection from clinical disease comparable to that obtained with two applications by using a conventional bacterin vaccine. In addition, immunized pigs were protected significantly from colonization of the lungs. This level of protective efficacy, which is a prerequisite to prevent an evolutionary increase of the intrinsic virulence levels of the pathogen (8), to date has only been obtained with a repeated intramuscular or aerosol application with bacterial ghosts (13, 15).



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  		FIG. 3. Lung lesion score (left) and humoral immune response (right) of naive and immunized pigs 7 days after homologous challenge. For immunization, a total of 6.5 x 105 A. pleuropneumoniae C5934{Delta}ureC{Delta}apxIIA were aerosolized; for infection, a total of 1.6 x 105 A. pleuropneumoniae C5934wt were used. Lung lesion scores were determined as described by Hannan et al. (12), and the bacteriological examination was done as previously described (2); mortality (Mo) and reisolation frequency of the A. pleuropneumoniae wild-type strain from lung tissue (RI) results for the different groups are given. The antibody response was assessed with two ELISAs with a detergent extract (solid symbols) and the recombinant ApxIIA protein (open symbols) as the solid-phase antigen. The response was quantified as a serum titer in comparison to an internal negative control (detergent extract ELISA) and in EU (based on an external standard) in the ApxIIA ELISA. For the standardized ApxIIA ELISA serum activities of <=10 EU are considered negative, 11 to 25 EU is intermediate, and >25 EU is a positive result. The central square within the hourglass shape represents the geometric mean, the hinges present the values in the middle of each half of data, and the top and bottom squares mark the maximum and minimum values, respectively. The asterisks denote statistical significance (P < 0.05) for the lung lesion score (Wilcoxon test) and the reisolation rate ({chi}2 test).


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ACKNOWLEDGMENTS
 
This work was supported by grant GE522/3-2 from the Deutsche Forschungsgemeinschaft, Bonn, Germany. W.T. is a fellow of the Mahanakorn University of Technology in Thailand.


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FOOTNOTES
 
* Corresponding author. Mailing address: Tieraerztliche Hochschule, Hannover Institut fuer Mikrobiologie und Tierseuchen, D-30173 Hanover, Germany. Phone: 49-511-856-7598. Fax: 49-511-856-7697. E-mail: gfgerlach{at}gmx.de. Back

Editor: B. B. Finlay

{dagger} Present address: Department of Microbiology, Faculty of Veterinary Medicine, Mahanakorn University of Technology, Bangkok, Thailand. Back


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Infection and Immunity, December 2002, p. 7120-7125, Vol. 70, No. 12
0019-9567/02/$04.00+0     DOI: 10.1128/IAI.70.12.7120-7125.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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