Convalescent pigs are protected completely against infection with a homologous Actinobacillus pleuropneumoniae strain but incompletely against a heterologous-serotype strain

Actinobacillus pleuropneumoniae: The molecular determinants of virulence and pathogenesis

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, is responsible for high economic losses in swine herds across the globe. Pleuropneumonia is characterized by severe respiratory distress and high mortality. The knowledge about the interaction between bacterium and host within the porcine respiratory tract has improved significantly in recent years. A. pleuropneumoniae expresses multiple virulence factors, which are required for colonization, immune clearance, and tissue damage. Although vaccines are used to protect swine herds against A. pleuropneumoniae infection, they do not offer complete coverage, and often only protect against the serovar, or serovars, used to prepare the vaccine. This review will summarize the role of individual A. pleuropneumoniae virulence factors that are required during key stages of pathogenesis and disease progression, and highlight progress made toward developing effective and broadly protective vaccines against an organism of great importance to global agriculture and food production.

Sero- and apx-typing of German Actinobacillus pleuropneumoniae field isolates from 2010 to 2019 reveals a predominance of serovar 2 with regular apx-profile

Serotyping is the most common method to characterize field isolates of Actinobacillus (A.) pleuropneumoniae, the etiological agent of porcine pleuropneumonia. Based on serology, many farms seem to be infected and antibodies against a wide variety of serovars are detectable, but, so far it is unknown to what degree respective serovars contribute to outbreaks of clinical manifest disease. In this study, 213 German A. pleuropneumoniae field isolates retrieved for diagnostic purposes from outbreaks of porcine pleuropneumonia between 2010 and 2019 were genetically serotyped and analyzed regarding their apx-toxin gene profile using molecular methods. Serotyping revealed a prominent role of serovar 2 in clinical cases (64% of all isolates) and an increase in the detection of this serovar since 2010 in German isolates. Serovar 9/11 followed as the second most frequent serovar with about 15% of the isolates. Furthermore, very recently described serovars 16 (n = 2) and 18 (n = 8) were detected. Most isolates (93.4%) showed apx-profiles typical for the respective serovar. However, this does not hold true for isolates of serovar 18, as 75% (n = 6) of all isolates of this serovar deviated uniformly from the “typical” apx-gene profile of the reference strain 7311555. Notably, isolates from systemic lesions such as joints or meninges did not harbor the complete apxICABD operon which is considered typical for highly virulent strains. Furthermore, the extremely low occurrence (n = 1) of NAD independent (biovar II) isolates in German A. pleuropneumoniae was evident in our collection of clinical isolates.

In Vivo Effectiveness of Injectable Antibiotics on the Recovery of Acute Actinobacillus pleuropneumoniae-Infected Pigs

The aim of this trial was to evaluate the in vivo effectiveness of injectable antibiotics of one- or two-dose administration on recovery of acute App (Actinobacillus pleuropneumoniae)-infected pigs. Ninety pigs with moderate general clinical score (GCS) of a commercial farm, suffering from acute App infection, were divided in two groups: (a) T1: one administration of gamithromycin injectable solution and (b) T2: two administrations of florfenicol injectable solution. Morbidity/mortality, clinical scores (clinical appearance score index-CAS, clinical respiratory score-CRS, clinical cough score index [CCS], general respiratory clinical score-GCRS, and general clinical score-GCS), body temperature score (BTS), and posttreatment interval were recorded. The carcass weight and lung scoring were estimated, based on slaughterhouse pleurisy evaluation system score, lung lobes score, and pneumonia area. The results of this study indicated that the tested antibiotics are efficacious for the recovery of acute App-affected pigs. Quicker improvement of BTS in sick pigs (at day 1 and 2) and quicker recovery of clinical signs, based on the improvement of clinical parameters (CAS, CCS, GCRS, GCS on day 2 and 3, and CRS on day 2), were noticed in T1 group. In conclusion, the use of tested antibiotics in acute App-affected pigs is an effective strategy for the control of an acute outbreak.

Complete Genome Sequence of Actinobacillus pleuropneumoniae Strain KL 16 (Serotype 1)

Actinobacillus pleuropneumoniae is a bacterial pathogen causing highly contagious porcine pleuropneumonia. Due to limited information on this species, it is difficult to study the biology of A. pleuropneumoniae at the genome level. Here, we report the fully annotated genome sequence of A. pleuropneumoniae strain KL 16.

B cell cross-epitope of Propionibacterium acnes and Actinobacillus pleuropneumonia selected by phage display library can efficiently protect from Actinobacillus pleuropneumonia infection

Contagious porcine pleuropneumonia (CPP), caused by Actinobacillus pleuropneumoniae (APP), is a highly transmissible and fatal respiratory illness that causes tremendous economic losses for the pig breeding industry worldwide. Propionibacterium acnes (PA) has a strong cross-reaction with anti-APP1 and anti-APP5 serum and can efficiently prevent APP infection, which was fortuitously found in researching the differential gene between the different APP serotypes. There seems to be some natural cross-protection between PA and APP. To identify the common epitope, the phage display library of a PA whole genome was constructed, whose size is 10⁵. The DNA sequence of the positive clone was determined after three rounds of biopanning, and ten common protein types were identified and the epitope was predicted by computer software. Six peptide epitopes were selected and synthesized for further analysis. Among these epitopes, Ba1, Bb5 and C1 could bind to anti-PA serum and anti-APP1 serum and vice versa. Furthermore, the IgG and IL-4 levels and CD4⁺/CD8⁺ T cell ratios in the Ba1, Bb5 and C1 groups were significantly higher than that in the control group, indicating that the epitopes could trigger an immune response, which was mainly humoral immunity. Moreover, Ba1 and Bb5 equally protected 80% of mice from a fatal dose of APP1 infection compared with the control group. Mice could resist APP1 and APP5 challenge after being treated with the combination of Ba1 and Bb5, with survival rates of 80% and 90%, respectively. These findings suggest that the PA epitope confers antigenicity and can heterologously resist to the APP infection. This finding provides a novel strategy for preventing APP infection.

Macrophages largely contribute to heterologous anti-Propionibacterium acnes antibody-mediated protection from Actinobacillus pleuropneumoniae infection in mice

Actinobacillus pleuropneumoniae is the causative agent of acute and chronic pleuropneumonia. Propionibacterium acnes is a facultative anaerobic gram-positive corynebacterium. We have previously found that anti-P. acnes antibodies can prevent A. pleuropneumoniae infections in mice. To investigate the role of macrophages in this process, affinity-purified anti-P. acnes IgG and anti-A. pleuropneumoniae IgG were used in opsonophagocytosis assays. Additionally, the efficacy of passive immunization with P. acnes serum against A. pleuropneumoniae was tested in macrophage-depleted mice. It was found that anti-P. acnes IgG had an effect similar to that of anti-A. pleuropneumoniae IgG (P > 0.05), which significantly promotes phagocytosis of A. pleuropneumoniae by macrophages (P < 0.01). It was also demonstrated that, after passive immunization with anti-P. acnes serum, macrophage-replete mice had the highest survival rate (90%), whereas the survival rate of macrophage-depleted mice was only 40% (P < 0.05). However, macrophage-depleted mice that had been passively immunized with naïve serum had the lowest survival rate (20%), this rate being lower than that of macrophage-replete mice that had been passively immunized with naïve serum. Overall, anti-P. acnes antibodies did not prevent A. pleuropneumoniae infection under conditions of macrophage depletion (P > 0.05). Furthermore, in mice that had been passively immunized with anti-P. acnes serum, macrophage depletion resulted in a greater A. pleuropneumoniae burden and more severe pathological features of pneumonia in lung tissues than occurred in macrophage-replete mice. It was concluded that macrophages are essential for the process by which anti-P. acnes antibody prevents A. pleuropneumoniae infection in mice.

Specific humoral immune response induced by propionibacterium acnes can prevent Actinobacillus pleuropneumoniae infection in mice

Porcine contagious pleuropneumonia, caused by Actinobacillus pleuropneumoniae, has a major impact on economics, ecology, and animal welfare in the pig-rearing industry. Propionibacterium acnes, a facultative anaerobic Gram-positive corynebacterium, exists widely in normal healthy adult animals. We have shown previously that P. acnes can prevent A. pleuropneumoniae infections in mice and pigs. To elucidate the mechanism of this effect and to identify novel A. pleuropneumoniae vaccines, the role of anti-P. acnes antibodies in preventing infection was analyzed by indirect immunofluorescence and opsonophagocytosis assays in vitro. The role of the specific humoral immune response induced by P. acnes was confirmed in a B cell depletion mouse model. The survival rates of mice challenged with A. pleuropneumoniae exhibited a highly significant positive rank correlation with the levels of anti-P. acnes antibodies. The specific antibodies induced by P. acnes had the ability to combine with A. pleuropneumoniae and increase opsonization of A. pleuropneumoniae for phagocytosis. Furthermore, analysis in the murine B cell depletion model confirmed that the humoral immune response induced by P. acnes played an important role in resistance to A. pleuropneumoniae infection. In this study, we further elucidated the reasons that P. acnes can prevent A. pleuropneumoniae infection, which provides useful evidence for the development of heterologous vaccines for the control of porcine contagious pleuropneumonia.

Association between transmission rate and disease severity for Actinobacillus pleuropneumoniae infection in pigs

A better understanding of the variation in infectivity and its relation with clinical signs may help to improve measures to control and prevent (clinical) outbreaks of diseases. Here we investigated the role of disease severity on infectivity and transmission of Actinobacillus pleuropneumoniae, a bacterium causing respiratory problems in pig farms. We carried out transmission experiments with 10 pairs of caesarean-derived, colostrum-deprived pigs. In each pair, one pig was inoculated intranasally with 5 × 10⁶ CFUs of A. pleuropneumoniae strain 1536 and housed together with a contact pig. Clinical signs were scored and the course of infection was observed by bacterial examination and qPCR analysis of tonsillar brush and nasal swab samples. In 6 out of 10 pairs transmission to contact pigs was observed, but disease scores in contact infected pigs were low compared to the score in inoculated pigs. Whereas disease score was positively associated with bacterial load in inoculated pigs and bacterial load with the transmission rate, the disease score had a negative association with transmission. These findings indicate that in pigs with equal bacterial load, those with higher clinical scores transmit A. pleuropneumoniae less efficiently. Finally, the correlation between disease score in inoculated pigs and in positive contact pigs was low. Although translation of experimental work towards farm level has limitations, our results suggest that clinical outbreaks of A. pleuropneumoniae are unlikely to be caused only by spread of the pathogen by clinically diseased pigs, but may rather be the result of development of clinical signs in already infected pigs.

Type IV fimbrial subunit protein ApfA contributes to protection against porcine pleuropneumonia

Porcine pleuropneumonia caused by Actinobacillus pleuropneumoniae accounts for serious economic losses in the pig farming industry worldwide. We examined here the immunogenicity and protective efficacy of the recombinant type IV fimbrial subunit protein ApfA as a single antigen vaccine against pleuropneumonia, or as a component of a multi-antigen preparation comprising five other recombinant antigens derived from key virulence factors of A. pleuropneumoniae (ApxIA, ApxIIA, ApxIIIA, ApxIVA and TbpB). Immunization of pigs with recombinant ApfA alone induced high levels of specific serum antibodies and provided partial protection against challenge with the heterologous A. pleuropneumoniae serotype 9 strain. This protection was higher than that engendered by vaccination with rApxIVA or rTbpB alone and similar to that observed after immunization with the tri-antigen combination of rApxIA, rApxIIA and rApxIIIA. In addition, rApfA improved the vaccination potential of the penta-antigen mixture of rApxIA, rApxIIA, rApxIIIA, rApxIVA and rTbpB proteins, where the hexa-antigen vaccine containing rApfA conferred a high level of protection on pigs against the disease. Moreover, when rApfA was used for vaccination alone or in combination with other antigens, such immunization reduced the number of pigs colonized with the challenge strain. These results indicate that ApfA could be a valuable component of an efficient subunit vaccine for the prevention of porcine pleuropneumonia.

Attenuated Actinobacillus pleuropneumoniae as a bacterial vector for expression of Mycoplasma hyopneumoniae P36 gene

BACKGROUND

Actinobacillus pleuropneumoniae and Mycoplasma hyopneumoniae are causative agents of porcine pneumonia. Over the last few years, attenuated A. pleuropneumoniae live vaccines have been shown to provide protection against A. pleuropneumoniae infection. We postulated that attenuated A. pleuropneumoniae could additionally be used as a vaccine vector for protection against M. hyopneumoniae.

METHODS

A mutant strain of A. pleuropneumoniae, SLW36, was constructed by replacing the urease structural gene of mutant strain SLW03 of A. pleuropneumoniae with the L-lactate dehydrogenase gene (p36) of M. hyopneumoniae by transconjugation and counter selection. The urease function and the growth kinetics of SLW36 were measured. Protein expression of P36 was analyzed by sodium dodecylsulfate-polyacrylamide gel electrophoresis and western blotting. The attenuated virulence and immunity of SLW36 were analyzed in a mouse model.

RESULTS

The mutant strain SLW36 was urease negative and four-fold less virulent than the parental strain SLW03. There were no differences in expression levels of p36 at different culture time-points and the foreign gene was stable after in vitro passage. Immunoglobulin G responses against p36 antigen and M. hyopneumoniae whole-cell antigen were detected.

CONCLUSIONS

The mutant strain SLW36 can induce antibody against p36 and M. hyopneumoniae. The mutant strain SLW36 has the potential to be used as a live vaccine for protection against A. pleuropneumoniae and M. hyopneumoniae. Studies in pigs are needed to confirm protective levels of antibodies and to check for rare side-effects of the vaccine.

Proteomic and immunoproteomic characterization of a DIVA subunit vaccine against Actinobacillus pleuropneumoniae

Background

Protection of pigs by vaccination against Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, is hampered by the presence of 15 different serotypes. A DIVA subunit vaccine comprised of detergent-released proteins from A. pleuropneumoniae serotypes 1, 2 and 5 has been developed and shown to protect pigs from clinical symptoms upon homologous and heterologous challenge. This vaccine has not been characterized in-depth so far. Thus we performed i) mass spectrometry in order to identify the exact protein content of the vaccine and ii) cross-serotype 2-D immunoblotting in order to discover cross-reactive antigens. By these approaches we expected to gain results enabling us to argue about the reasons for the efficacy of the analyzed vaccine.

Results

We identified 75 different proteins in the vaccine. Using the PSORTb algorithm these proteins were classified according to their cellular localization. Highly enriched proteins are outer membrane-associated lipoproteins like OmlA and TbpB, integral outer membrane proteins like FrpB, TbpA, OmpA1, OmpA2, HgbA and OmpP2, and secreted Apx toxins. The subunit vaccine also contained large amounts of the ApxIVA toxin so far thought to be expressed only during infection. Applying two-dimensional difference gel electrophoresis (2-D DIGE) we showed different isoforms and variations in expression levels of several proteins among the strains used for vaccine production. For detection of cross-reactive antigens we used detergent released proteins of serotype 7. Sera of pigs vaccinated with the detergent-released proteins of serotypes 1, 2, and 5 detected seven different proteins of serotype 7, and convalescent sera of pigs surviving experimental infection with serotype 7 reacted with 13 different proteins of the detergent-released proteins of A. pleuropneumoniae serotypes 1, 2, and 5.

Conclusions

A detergent extraction-based subunit vaccine of A. pleuropneumoniae was characterized by mass spectrometry. It contained a large variety of immunogenic and virulence associated proteins, among them the ApxIVA toxin. The identification of differences in expression as well as isoform variation between the serotypes implied the importance of combining proteins of different serotypes for vaccine generation. This finding was supported by immunoblotting showing the induction of cross-reactive antibodies against several surface associated proteins in immunized animals.

Disease patterns and immune responses in the offspring to sows with high or low antibody levels to Actinobacillus pleuropneumoniae serotype 2

The serum antibody responses to Actinobacillus pleuropneumoniae and the secondary invader Pasteurella multocida were monitored from birth until slaughter in the offspring to sows with high or low levels of serum antibodies to A. pleuropneumoniae. Serum antibody concentrations to A. pleuropneumoniae were higher from birth to the age of 9 weeks in piglets delivered by high responding sows. In contrast, antibody levels to P. multocida were similar in both groups during this period. From the age of 20 and 15 weeks, antibody levels to A. pleuropneumoniae and P. multocida, respectively, were higher in the offspring to high responding sows. This implies that the offspring to sows with high levels of antibodies may be better protected during the first period of life because of a higher level of passively derived immunity. These piglets will also mount a higher antibody response when later infected, indicating a heritability of the humoral immune response.

Field experience with two different vaccination strategies aiming to control infections with Actinobacillus pleuropneumoniae in a fattening pig herd

Background

The prevalence of pleurisies recorded at slaughter is increasing in Sweden, and acute outbreaks of actinobacillosis that require antimicrobial treatments have become more frequent. As an increased use of antimicrobials may result in the development of antimicrobial resistance it is essential to develop alternative measures to control the disease. Vaccinations present an appealing alternative to antimicrobial treatments. The aim of this work was to evaluate the potential of two different vaccination strategies in a specialized fattening herd affected by actinobacillosis.

Methods

The study was conducted in a specialized fattening herd employing age segregated rearing in eight units. The herd suffered from infections caused by Actinobacillus pleuropneumoniae serotype 2, confirmed by necropsy and serology. The study included 54 batches of pigs grouped into five periods. Batches of pigs of the second period were vaccinated against actinobacillosis twice, and pigs in the fourth period were vaccinated three times. Batches of pigs of the first, third and fifth period were not vaccinated. Concentrations of serum antibodies to A. pleuropneumoniae and serum amyloid A (SAA) were analysed and production data were recorded.

Results

Despite vaccinating, medical treatments were required to reduce the impact of the disease. The mean incidence of individual treatments for respiratory diseases during the rearing period ranged from 0 to 4.7 ± 1.8%, and was greatest during the triple vaccination period (period IV; p < 0.05 when compared to other groups). A large proportion of the vaccinated pigs seroconverted to A. pleuropneumoniae serotype 2 in the absence of a SAA-response. The prevalence of pleuritis decreased from 25.4 ± 6.5% in the first period to 5.0 ± 3.7% in the fifth period (p < 0.001).

Conclusions

The vaccine did not effectively prevent clinical expression of A. pleuropneumoniae infections, but seroconversion to A. pleuropneumoniae in the absence of a SAA-response in a large number pigs indicated that the vaccine had activated the immune system. Further, the prevalence of pleuritis decreased with time. This indicates that vaccinations together with intensified medical treatments of affected pigs could be useful in reducing the impact of A. pleuropneumoniae serotype 2 infections.

Genomic differences between Actinobacillus pleuropneumoniae serotypes 1 and 3 and the diversity distribution among 15 serotypes

The limited information on the genetic differences among the 15 currently known serotypes of Actinobacillus pleuropneumoniae has significantly hampered the development of typing-based diagnostic methods and multivalent vaccines. In this study, we compared the genomic differences between A. pleuropneumoniae strains CVCC259 (serotype 1) and CVCC261 (serotype 3) by representational difference analysis. Of the eight differential DNA sequences in the CVCC259 strain and 11 differential DNA sequences in the CVCC261 strain that we identified, seven represent known virulent genes, 10 encode putative proteins, and two encode hypothetical proteins. We also investigated the distribution of these 19 sequences among the 15 serotypes, and each serotype showed a different distribution pattern. The autotransporter adhesin occurred as a novel putative virulence factor in serotypes 1, 5, 7, 8, 9, and 11. Notably, the presence of wzm and wzt in serotypes 1, 9, and 11 and the diverse distribution of wzz and wzy in the other serotypes suggest the presence of different O-antigen biosynthesis pathways among serotypes. The information on the differential distribution of these DNA sequences in the 15 serotypes of A. pleuropneumoniae may contribute to future research on the pathogenic mechanisms of different serotypes, typing-based diagnosis methods, and multivalent vaccines.

Responses of pigs to a re-challenge with Actinobacillus pleuropneumoniae after being treated with different antimicrobials following their initial exposure

Four groups of six specific pathogen-free (SPF) pigs were inoculated intranasally with Actinobacillus pleuropneumoniae serotype 2 and treated with either enrofloxacin, tetracycline or penicillin at the onset of clinical disease, or left untreated. A fifth group was left uninoculated. The inoculated control and the penicillin-treated groups developed severe disease, but the groups treated with enrofloxacin and tetracycline recovered rapidly. All the inoculated pigs, except those treated with enrofloxacin developed serum antibodies to A pleuropneumoniae. On day 28, all five groups were challenged with A pleuropneumoniae without any subsequent treatment. The previously uninoculated control group and the enrofloxacin-treated group developed severe disease, but the three seropositive groups remained unaffected.

Screening of strain-specific Actinobacillus pleuropneumoniae genes using a combination method

We describe a three-step method designed to identify distinct antigen-coding genes between two related bacterial genomes by: (a) constructing a subtractive library using Representational Difference Analysis (RDA), (b) characterization of gene expression in vitro using a ribosome display system combined with antibody screening and (c) gene recovery and confirmation using RT-PCR and reverse Southern hybridization, respectively. To test the efficacy of this strategy we screened the antigen-coding gene profile of Actinobacillus pleuropneumoniae (APP) strains CCVC259 and CCVC263 that do not elicit cross-protective immunity. This strategy identified six different DNA fragments from CCVC259 and 10 different DNA fragments from CCVC263. Of six sequences identified from CCVC259, 2 were not significantly similar, two were 74% and 87% homologous to the sequences encoding for the Ralstonia eutropha H16 conserved membrane protein and transcriptional regulator respectively, and two were >96% homologous to the Pseudomonas alcaligenes putative transposase subunit genes IS1474 and IS1475. Among ten unique DNA fragments identified from strain CCVC263, eight were homologous to DNA fragments encoding the TBP 1 precursor, ATP-dependent helicase HepA, glycosylase, methyltransferase and GTPase in the APP L20 genome and two genes identified had no significant similarity. Our findings indicated that the three-step method could be utilized to identify unique antigen-coding genes and may be a powerful and efficient technique for serotype-specific identification of pathogens and polyvalent vaccine design.

Selection of serotype-specific vaccine candidate genes in Actinobacillus pleuropneumoniae and heterologous immunization with Propionibacterium acnes

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is a highly contagious lethal causative agent of swine pleuropneumoniae. Vaccines for this disease are usually serotype specific. In order to identify immunogenic genes specific to serotypes, two differentially expressed gene cDNA libraries of A. pleuropneumoniae CCVC259 (serotype 1) and CCVC263 (serotype 5) had been constructed by using a cDNA representational difference analysis (cDNA-RDA). From the libraries, six potential vaccine candidate genes expressed only in serotype 1 and 13 genes in serotype 5 were identified by antibody screening after gene expression in vitro with a ribosome display system. Eight sequences out of these exhibited 77-100% identity to the corresponding genes in Propionibacterium acnes. The antisera raised against A. pleuropneumoniae serotypes 1 and 5 were reactive with P. acnes at a titer of 1:6400 and vice versa (ELISA titer, 1:3200). Mice immunized with P. acnes were protected against 10 x LD50 challenge with A. pleuropneumoniae serotypes 1 and 5, and the survival rates were 90% and 95%, respectively. Pigs vaccinated with the P. acnes strain could develop high level antibody cross-reacted with A. pleuropneumoniae and obtain noticeable protection from A. pleuropneumoniae infection. These data demonstrate that there were common antigens between A. pleuropneumoniae and P. acnes, and the cross protectivity highlights the possibility of using P. acnes vaccines for preventing infection by A. pleuropneumoniae.

Actinobacillus pleuropneumoniaevaccines: from bacterins to new insights into vaccination strategies

With the growing emergence of antibiotic resistance and rising consumer demands concerning food safety, vaccination to prevent bacterial infections is of increasing relevance.Actinobacillus pleuropneumoniaeis the etiological agent of porcine pleuropneumonia, a respiratory disease leading to severe economic losses in the swine industry. Despite all the research and trials that were performed withA. pleuropneumoniaevaccination in the past, a safe vaccine that offers complete protection against all serotypes has yet not reached the market. However, recent advances made in the identification of new potential vaccine candidates and in the targeting of specific immune responses, give encouraging vaccination perspectives. Here, we review past and current knowledge onA. pleuropneumoniaevaccines as well as the newly available genomic tools and vaccination strategies that could be useful in the design of an efficient vaccine againstA. pleuropneumoniaeinfection.

Potential use an Actinobacillus pleuropneumoniae double mutant strain ΔapxIICΔapxIVA as live vaccine that allows serological differentiation between vaccinated and infected animals

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a highly contagious and often fatal disease. We have previously reported the construction and characterization of a single gene apxIIC deletion mutant HB04C(-) based on A. pleuropneumoniae serovar 7 which produces ApxII toxin and ApxIV. A precisely defined DeltaapxIICDeltaapxIVA double-deletion mutant of A. pleuropneumoniae was constructed based on HB04C(-) by transconjugation and counterselection, and the levels of virulence of the DeltaapxIIC single mutant and DeltaapxIICDeltaapxIVA double mutant were compared in an experimental infection in mice and pigs. The results demonstrated that the DeltaapxIICDeltaapxIVA double mutant strain was less virulent than HB04C(-). Despite attenuation of virulence, the DeltaapxIICDeltaapxIVA double mutant remains immunogenic and conferred a similar level of protective immunity to pigs against challenge with a lethal dose of a heterologous fully virulent standard serovar 1 strain of A. pleuropneumoniae. The results of the virulence study suggest that ApxIV is a critical virulence factor of A. pleuropneumoniae serovar 7 and is able to induce clinical disease, but it not required for efficient vaccination of pigs against A. pleuropneumoniae infection. Two weeks after the booster immunization, animals vaccinated with HB04C(-) were positive in the ApxIVAM-ELISA based on a recombinant GST-fusion protein GST-ApxIVAM as the solid-phase antigen while animals vaccinated with the DeltaapxIICDeltaapxIVA double mutant were negative. These data demonstrate that the double mutant DeltaapxIICDeltaapxIVA can be used as an effective live marker vaccine allowing serological differentiation between vaccinated and infected animals.

Use of an Actinobacillus pleuropneumoniae multiple mutant as a vaccine that allows differentiation of vaccinated and infected animals

Vaccination against Actinobacillus pleuropneumoniae is hampered by the lack of vaccines inducing reliable cross-serotype protection. In contrast, pigs surviving natural infection are at least partially protected from clinical symptoms upon reinfection with any serotype. Thus, we set out to construct an attenuated A. pleuropneumoniae live vaccine allowing the differentiation of vaccinated from infected animals (the DIVA concept) by successively deleting virulence-associated genes. Based on an A. pleuropneumoniae serotype 2 prototype live negative marker vaccine (W. Tonpitak, N. Baltes, I. Hennig-Pauka, and G.-F. Gerlach, Infect. Immun. 70:7120-7125, 2002), genes encoding three enzymes involved in anaerobic respiration and the ferric uptake regulator Fur were deleted, resulting in a highly attenuated sixfold mutant; this mutant was still able to colonize the lower respiratory tract and induced a detectable immune response. Upon a single aerosol application, this mutant provided significant protection from clinical symptoms upon heterologous infection with an antigenically distinct A. pleuropneumoniae serotype 9 challenge strain and allowed the serological discrimination between infected and vaccinated groups.

Identification of genes transcribed by Actinobacillus pleuropneumoniae in necrotic porcine lung tissue by using selective capture of transcribed sequences

Genes expressed by Actinobacillus pleuropneumoniae in necrotic porcine lung tissue were identified by selective capture of transcribed sequences analysis. In total, 46 genes were identified, 20 of which have been previously reported to be associated with in vivo expression or virulence in A. pleuropneumoniae or in other organisms.

Characterization of the antigenic lipopolysaccharide O chain and the capsular polysaccharide produced by Actinobacillus pleuropneumoniae serotype 13

Serotyping of Actinobacillus pleuropneumoniae, the etiologic agent of porcine pleuropneumonia, is important for epidemiological studies and for the development of homologous vaccine cell preparations. The serology is based on the specific chemical structures of capsular polysaccharides (CPSs) and lipopolysaccharide (LPS) antigenic O-polysaccharide moieties (O-PSs), and knowledge of these structures is required for a molecular-level understanding of their serological specificities. The structures of A. pleuropneumoniae serotype 1 to 12 CPSs and O-PSs have been elucidated; however, the structures associated with three newly proposed serotypes (serotypes 13, 14, and 15) have not been reported. Herein we described the structures of the antigenic O-PS and CPS of A. pleuropneumoniae serotype 13. The O-PS of the A. pleuropneumoniae serotype 13 LPS is a polymer of branched tetrasaccharide repeating units composed of l-rhamnose, 2-acetamido-2-deoxy-d-galactose, and d-galactose residues (1:1:2). By use of hydrolysis, methylation, and periodate oxidation chemical methods together with the application of one- and two-dimensional 1H and 13C nuclear magnetic resonance spectroscopy and mass spectrometry, the structures of the O chain and CPS were determined. The CPS of A. pleuropneumoniae serotype 13 was characterized as a teichoic-acid type polymer. The LPS O antigen was identical to the O-PS produced by A. pleuropneumoniae serotype 7. The CPS has the unique structure of a 1,3-poly(glycerol phosphate) teichoic acid type I polymer and constitutes the macromolecule defining the A. pleuropneumoniae serotype 13 antigenic specificity.

Comparison of the efficacy of a subunit and a live streptomycin-dependent porcine pleuropneumonia vaccine

OBJECTIVE

To evaluate the efficacy of two new-generation porcine pleuropneumonia vaccines when challenged with Australian isolates of Actinobacillus pleuropneumoniae of serovars 1 and 15.

DESIGN

The Porcilis APP vaccine and an experimental streptomycin-dependent strain of A pleuropneumoniae were evaluated in a standardised pen trial. Each vaccine/challenge group consisted of 10 pigs.

RESULTS

With the serovar 1 challenge, the Porcilis APP vaccine and the live vaccine, compared with the control group, gave significant protection in terms of clinical signs, lung lesions, re-isolation scores and average daily gain (ADG) postchallenge. Only the Porcilis APP vaccine provided significant protection against mortality. In the serovar 15 challenged pigs, the only significant difference detected was that the Porcilis APP vaccinated pigs had a better postchallenge ADG than the controls. None of the Porcilis APP vaccinated pigs showed signs of depression postvaccination and none were euthanased after challenge with either serovar 1 or 15. The pigs vaccinated with the live vaccine showed obvious depression after each vaccination and a total of 3 pigs were euthanased after challenge (one with serovar 1 and two with serovar 15).

CONCLUSIONS

Both of the vaccines provided significant protection against a severe challenge with serovar 1 A pleuropneumoniae. Neither vaccine was effective against a serovar 15 A pleuropneumoniae challenge. There was evidence that the Porcilis APP vaccine did provide some protection against the serovar 15 challenge because the ADG, after challenge of pigs given this vaccine, was greater than the control pigs.

Identification of Actinobacillus pleuropneumoniae genes important for survival during infection in its natural host

Actinobacillus pleuropneumoniae is a strict respiratory tract pathogen of swine and is the causative agent of porcine pleuropneumonia. We have used signature-tagged mutagenesis (STM) to identify genes required for survival of the organism within the pig. A total of 2,064 signature-tagged Tn10 transposon mutants were assembled into pools of 48 each, and used to inoculate pigs by the endotracheal route. Out of 105 mutants that were consistently attenuated in vivo, only 11 mutants showed a >2-fold reduction in growth in vitro compared to the wild type, whereas 8 of 14 mutants tested showed significant levels of attenuation in pig as evidenced from competitive index experiments. Inverse PCR was used to generate DNA sequence of the chromosomal domains flanking each transposon insertion. Only one sibling pair of mutants was identified, but three apparent transposon insertion hot spots were found--an anticipated consequence of the use of a Tn10-based system. Transposon insertions were found within 55 different loci, and similarity (BLAST) searching identified possible analogues or homologues for all but four of these. Matches included proteins putatively involved in metabolism and transport of various nutrients or unknown substances, in stress responses, in gene regulation, and in the production of cell surface components. Ten of the sequences have homology with genes involved in lipopolysaccharide and capsule production. The results highlight the importance of genes involved in energy metabolism, nutrient uptake and stress responses for the survival of A. pleuropneumoniae in its natural host: the pig.

Construction of an Actinobacillus pleuropneumoniae serotype 2 prototype live negative-marker vaccine

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.

Actinobacillus pleuropneumoniae serotype 1 carrying the defined aroA mutation is fully avirulent in the pig

The aroA gene from Actinobacillus pleuropneumoniae serotype 1 reference strain 4074 was isolated and sequenced. The gene complemented the aroA mutation in Escherichia coli AB2829. A kanamycin resistance cassette was inserted into the aroA gene and the mutant gene was reintroduced into A. pleuropneumoniae by allelic replacement. Intratracheal infection of susceptible pigs with A. pleuropneumoniae aroA caused no signs of respiratory disease or lung lesions in any of the animals at a dose 10(4) times the dose reliably known to induce acute pleuropneumonia; all animals infected with the unaltered control strain developed acute disease. The aroA mutant was rapidly eliminated from the lungs and tonsil of infected animals. The mutant may represent a safely attenuated strain for use in live bacterial vaccination or the delivery of antigen by the intranasal route. However, the residence time of the mutant in the respiratory tract of the pig may be too short for it to be useful in generating a protective immune response.

Actinobacillus pleuropneumoniae: pathobiology and pathogenesis of infection

Actinobacillus pleuropneumoniae causes porcine pleuropneumonia, a highly contagious disease for which there is no effective vaccine. This review considers how adhesins, iron-acquisition factors, capsule and lipopolysaccharide, RTX cytotoxins and other potential future vaccine components contribute to colonisation, to avoidance of host clearance mechanisms and to damage of host tissues.

Actinobacillus pleuropneumoniae surface polysaccharides: their role in diagnosis and immunogenicity

Actinobacillus pleuropneumoniae is an important pig pathogen that is responsible for swine pleuropneumonia, a highly contagious respiratory infection. Knowledge of the importance, composition and structural determination of the major antigens involved in virulence provides crucial information that could lead to the development of a rationale for the production of specific serodiagnostic tools as well as vaccine development. Thus, efforts have been devoted to study mainly A. pleuropneumoniae virulence determinants with special emphasis on the Apx toxins (for A. pleuropneumoniae RTX toxins). In comparison, little attention has been given to the surface polysaccharides, which include capsular polysaccharides (CPS) and cell-wall lipopolysaccharides (LPS). Here, we review current knowledge on CPS and LPS of A. pleuropneumoniae used as diagnostic tools to monitor the infection and as immunogens for inclusion in vaccine preparations for animal protection.

Improved protection against lung colonization by Actinobacillus pleuropneumoniae ghosts: characterization of a genetically inactivated vaccine

Pigs immunized with Actinobacillus pleuropneumoniae ghosts or a formalin-inactivated bacterin were found to be protected against clinical disease in both vaccination groups, whereas colonization of the lungs with A. pleuropneumoniae was only prevented in ghost-vaccinated pigs. Bacterial ghosts are empty cell envelopes created by the expression of a cloned bacteriophage lysis gene and, unlike formalin-inactivated bacteria, suffer no denaturing steps during their production. This quality may lead to a superior presentation of surface antigens to the immune system. Analysis by SDS-PAGE and immunoblotting of the two vaccine preparations revealed different contents of antigenic proteins. In order to better understand the immunogenic properties of A. pleuropneumoniae ghosts and formalin-inactivated bacteria, we compared the serum antibody response induced in both treatment groups. Immune sera were tested on whole cell antigen or purified virulence factors including outer membrane protein preparations (OMPs), outer membrane lipoprotein OmlA1, transferrin binding proteins (TfbA1, TfbA7 and TfbB) and Apx toxins (ApxI, II and III). SDS-PAGE and immunoblots revealed no specific antibody response against the single virulence factors tested in any vaccinated animal. The two vaccination groups showed different recognition patterns of whole cell antigen and OMP-enriched preparations. A 100 kDa protein was recognized significantly stronger by ghost-vaccinated pigs than convalescent pigs. This unique antibody population induced by ghosts could play a determining role in the prevention of lung colonization. The same 100 kDa antigen was recognized by ghost-sera in homologous as well as heterologous serotype A. pleuropneumoniae protein preparations. Indications for a crossprotective potential in the ghost vaccine were supported by studies on rabbit hyperimmune sera.

Actinobacillus species and their role in animal disease

Actinobacillus species are Gram-negative bacteria responsible for several quite distinct disease conditions of animals. The natural habitat of the organisms is primarily the upper respiratory tract and oral cavity. A. lignieresii is the cause of actinomycosis (wooden tongue) in cattle: a sporadic, insidiously-developing granulomatous infection. In sharp contrast is A. pleuropneumoniae which is responsible for a rapidly spreading often fatal pneumonia, common among intensively reared pigs. Detailed investigation of this organism has provided a much clearer picture of the bacterial factors involved in causing disease. A. equuli similarly causes a potent septicaemia in the neonatal foal; growing apparently unrestricted once infection occurs. Other members of the genus induce characteristic pathogenesis in their preferred host, with one, A. actinomycetemcomitans, being a cause of human periodontal disease. This article reviews recent understanding of the taxonomy and bacteriology of the organisms, and the aetiology, pathogenicity, diagnosis and control of animal disease caused by Actinobacillus species.

Protective efficacy of an affinity-purified hemolysin vaccine against experimental swine pleuropneumonia

Protective efficacy of Actinobacillus pleuropneumoniae extracellular hemolytic toxins, RTX-toxin I (Apx I) and Apx II, was evaluated in pigs. The hemolysins were purified from culture supernatant of A. pleuropneumoniae strain HA-337, serotype 1 by immunoaffinity chromatography using a monoclonal antibody specific for Apx I or Apx II as ligand. Four pigs were vaccinated with the purified hemolysins absorbed on aluminum phosphate gel adjuvant. Four pigs of a control group were given placebo. Hemolysin-neutralizing antibodies were detected only in vaccinated group after booster injection. One of four control pigs died following an aerosol challenge with the homologous strain, and three surviving pigs developed serious clinical signs of pneumonia and had extensive lung lesions. In contrast, there was no mortality in vaccinated group. Only transient hyperthermia was observed in two vaccinated pigs after challenge. A necropsy, two vaccinated pigs had slight localized pulmonary lesions, though the remaining two had no lung lesions at all. These results indicate that the hemolysin vaccine made of Apx I and Apx II has good protective activity against swine pleuropneumonia caused by A. pleuropneumoniae serotype.

Cross-protection between Actinobacillus pleuropneumoniae biotypes-serotypes in pigs

Four groups of hysterectomy-derived and colostrum-deprived pigs were intranasally inoculated with an Actinobacillus pleuropneumoniae biotype 1-serotype 2 strain (producing RTX toxins ApxII and ApxIII. 6 pigs), an A. pleuropneumoniae biotype 1-serotype 10 strain (producing ApxI. 5 pigs), an A. pleuropneumoniae biotype 2-serotype 2 strain (producing ApxII, 5 pigs) or saline (controls, 7 pigs). All pigs were exposed to A. pleuropneumoniae biotype 1-serotype 2 endobronchial challenge. After challenge, severe clinical signs were observed in all control pigs, one pig immunized with the A. pleuropneumoniae biotype 1-serotype 10 strain and two pigs immunized with the A. pleuropneumoniae biotype 2-serotype 2 strain. These pigs died within 36 h after challenge and 20 to 50% of the lungs were macroscopically affected. In the other pigs, clinical signs were mild or absent and no or only small, focal lung lesions were observed when euthanized at 48 h after challenge. At the time challenge neutralizing antibodies against ApxI only. ApxII only and both ApxII and III were present in sera of pigs immunized with the A. pleuropneumoniae biotype 1-serotype 10 strain, the A. pleuropneumoniae biotype 2-serotype 2 strain and the A. pleuropneumoniae biotype 1-serotype 2 strain, respectively. These results indicate that immune mechanisms other than Apx neutralizing antibodies were involved in partial cross-protection of pigs immunized against A. pleuropneumoniae biotype 1-serotype 10 and challenged with the A. pleuropneumoniae biotype 1-serotype 2.