Development of a DIVA subunit vaccine against Actinobacillus pleuropneumoniae infection

Actinobacillus pleuropneumoniae, surface proteins and virulence: a review

Actinobacillus pleuropneumoniae (App) is a globally distributed Gram-negative bacterium that produces porcine pleuropneumonia. This highly contagious disease produces high morbidity and mortality in the swine industry. However, no effective vaccine exists to prevent it. The infection caused by App provokes characteristic lesions, such as edema, inflammation, hemorrhage, and necrosis, that involve different virulence factors. The colonization and invasion of host surfaces involved structures and proteins such as outer membrane vesicles (OMVs), pili, flagella, adhesins, outer membrane proteins (OMPs), also participates proteases, autotransporters, and lipoproteins. The recent findings on surface structures and proteins described in this review highlight them as potential immunogens for vaccine development.

Evaluation of the Protective Immune Response Induced by an rfbG-Deficient Salmonella enterica Serovar Enteritidis Strain as a Live Attenuated DIVA (Differentiation of Infected and Vaccinated Animals) Vaccine in Chickens

Salmonella enterica serovar Enteritidis (S. Enteritidis), one of the zoonotic pathogens, not only results in significant financial losses for the global poultry industry but also has the potential to spread to humans through poultry and poultry products. Vaccination is an effective method to prevent Salmonella infections. In this study, we constructed a live attenuated DIVA (differentiation of infected and vaccinated animals) vaccine candidate, Z11ΔrfbG, and evaluated its protective effectiveness and DIVA potential in chickens. Compared to that of the virulent wild-type strain, the 50% lethal dose (LD₅₀) of the rfbG mutant strain increased 56-fold, confirming its attenuation. High serum levels of S. Enteritidis-specific IgG titers indicated that a significant humoral immune response was induced in the vaccinated group. After challenge, the nonvaccinated group showed serious clinical symptoms (diarrhea, depression, decreased appetite, ruffled feathers, and weight loss), pathological changes (white nodules in the liver and fatty lesions in liver cells), and death. In contrast, there were no clinical symptoms, pathological changes, or death in the 5 × 10⁶- and 5 × 10⁷-CFU-vaccinated groups. Z11ΔrfbG vaccination significantly reduced S. Enteritidis colonization in the spleen, liver, and cecum. In addition, the Z11ΔrfbG-vaccinated group exhibited a negative response to the serological test, whereas the virulent wild-type Z11 infection group was strongly positive for the serological test, showing a DIVA capability of Z11ΔrfbG vaccination. Overall, our findings demonstrate the viability of the rfbG mutant as a live attenuated chicken vaccine that can discriminate between animals that have been immunized and those that have been infected. IMPORTANCE S. Enteritidis is a highly adapted pathogen that causes significant economic losses in the poultry industry around the world. Vaccination is an effective method of controlling S. Enteritidis infections. Here, we demonstrated that S. Enteritidis Z11ΔrfbG has the potential to be a safe, immunogenic, and DIVA vaccine candidate for the control of Salmonella infections in chickens. Z11ΔrfbG not only provided effective protection in chickens but also distinguished between infected and vaccinated chickens by serological tests.

A Combinatorial Vaccine Containing Inactivated Bacterin and Subunits Provides Protection Against Actinobacillus pleuropneumoniae Infection in Mice and Pigs

Actinobacillus pleuropneumoniae (APP) is the etiological agent of porcine contagious pleuropneumonia (PCP) that causes great economic losses in the swine industry. Currently, vaccination is still a commonly used strategy for the prevention of the disease. Commercially available vaccines of this disease, including inactivated bacterins and subunit vaccines, have clinical limitations such as side effects and low cross-protection. In this study, a combinatorial vaccine (Bac-sub) was developed, which contained inactivated bacterial cells of a serovar 1 strain and three recombinant protoxins (rApxIA, rApxIIA, and rApxIIIA). Its side effects, immune protection, and cross-protection were evaluated and compared with a commercial subunit vaccine and a commercial trivalent bacterin in a mouse infection model. The results revealed that the Bac-sub vaccine showed no obvious side effects, and induced higher levels of Apx toxin-specific IgG, IgG1, and IgG2a than the commercial vaccines after booster. After a challenge with virulent strains of serovars 1, 5, and 7, the Bac-sub vaccine provided greater protection (91.76%, 100%, and 100%, respectively) than commercial vaccines. Much lower lung bacterial loads (LBLs) and milder lung lesions were observed in the Bac-sub-vaccinated mice than in those vaccinated with the other two vaccines. The protective efficacy of the Bac-sub vaccine was further evaluated in pigs, which showed that vaccinated pigs displayed significantly milder clinical symptoms and lung lesions than the unvaccinated pigs after the challenge. Taken together, Bac-sub is a safe and effective vaccine that could provide high protection against A. pleuropneumoniae infection in both mice and pigs.

Candidate genes and gene markers for the resistance to porcine pleuropneumonia

Actinobacillus (A.) pleuropneumoniae is one of the most important respiratory pathogens in global pig production. Antimicrobial treatment and vaccination provide only limited protection, but genetic disease resistance is a very promising alternative for sustainable prophylaxis. Previous studies have discovered multiple QTL that may explain up to 30% of phenotypic variance. Based on these findings, the aim of the present study was to use genomic sequencing to identify genetic markers for resistance to pleuropneumonia in a segregating commercial German Landrace line. 163 pigs were infected with A. pleuropneumoniae Serotype 7 through a standardized aerosol infection method. Phenotypes were accurately defined on a clinical, pathological and microbiological basis. The 58 pigs with the most extreme phenotypes were genotyped by sequencing (next-generation sequencing). SNPs were used in a genome-wide association study. The study identified genome-wide associated SNPs on three chromosomes, two of which were chromosomes of QTL which had been mapped in a recent experiment. Each variant explained up to 20% of the total phenotypic variance. Combined, the three variants explained 52.8% of the variance. The SNPs are located in genes involved in the pathomechanism of pleuropneumonia. This study confirms the genetic background for the host's resistance to pleuropneumonia and indicates a potential role of three candidates on SSC2, SSC12 and SSC15. Favorable gene variants are segregating in commercial populations. Further work is needed to verify the results in a controlled study and to identify the functional QTN.

Immune response and protection against Lawsonia intracellularis infections in pigs

Lawsonia intracellularis are Gram-negative, obligate intracellular bacteria that cause proliferative enteropathy (PE), an economically important disease for the pig industry. Numerous reviews have been published on the characteristics and pathogenesis of this bacterium since its isolation and taxonomic characterization, with most reviews only partially covering how the host immune response develops during infection and the immune correlates of protection. With the development of increasingly more sophisticated immunological assays and tools for the pig, the immune response against L. intracellularis at distinct stages of pathogenesis has been published. In this review, we discuss current knowledge of the pig immune response against L. intracellularis and strategies to achieve immune protection. The immune response is presented in relation to chronological progression of pathological lesions and clinical symptoms, with emphasis on innate immunity and the adaptive humoral and cell-mediated immune response. The aim is to achieve a comprehensive understanding of the host immune response with respect to the stage-dependent cellular and biochemical processes important during PE development. Also, strategies for development of immune protection and new vaccination technologies are discussed in the light of new discoveries in the field.

Efficacy of bacteriophage Lysed Pasteurella marker vaccine in laboratory animal models with a novel DIVA for haemorrhagic septicaemia

OBJECTIVE

The present study aimed at evaluating the efficacy of an improved phage lysate marker vaccine for haemorrhagic septicaemia in mice and rabbit model and development of a DIVA ELISA based on iron restricted outer membrane protein (IROMP).

METHOD

The experimental vaccine was prepared by lysing P. multocida B:2 grown under iron restricted conditions with a Pasteurella bacteriophage and addition of an alum adjuvant to enhance the immunogenicity. The vaccine was administered in mice and rabbits divided into two group each. Phage lysate vaccine (PL-VacI) was administered to group I mice and rabbits whereas group II mice and rabbits received alum precipitated HS vaccine (HS-VacII). Antibody titres were monitored 0, 30, 60, 90, 210 and 240 dpv. An IROMP (130 kDa) based indirect ELISA was also developed to differentiate between infected and vaccinated animals. The Pasteurella phage isolated in present study was sequenced at Georgia Genomic Facilty, Georgia.

RESULT

The sequence of PMP-GAD-IND (Pasteurella bacteriophage) was deposited in GenBank under no KY203335. The group I mice and rabbits vaccinated with Phage lysate vaccine (PL-VacI) group revealed significantly higher antibody titres than group II mice and rabbits receiving alum-precipitated bacterin (HS-VacII) by MAT, IHA and ELISA (P < 0.05 and P < 0.001). The peak log 10 values (3.46) in case of group I mice by ELISA were attained at 90DPI whereas in group II mice the peak values at 90DPI were 2.82. Mean log10 titres by ELISA in group I and II rabbits were 2.43 and 2.35 respectively at 30DPI whereas at 120DPI the titres were 3.29 and 2.75, respectively. The DIVA ELISA detected presence of a novel 137 kDa IROMP/siderophore antibody in sera of group I mice and rabbits (PL-VacI) absent in sera of mice and rabbits given HS-VacII.

CONCLUSION

The bacteriophage based marker vaccine (PL-VacI) had a more effective and longer immune response against HS in mice and rabbit in comparison to the widely used alum precipitated HS vaccine (HS-VacII). Moreover, the development of a recombinant IROMP based indirect ELISA could serve as an excellent tool to differentiate between infected and vaccinated cattle and buffaloes for effective control of HS.

Polymorphism analysis of the apxIA gene of Actinobacillus pleuropneumoniae serovar 5 isolated in swine herds from Brazil

The bacterium Actinobacillus pleuropneumoniae is the etiological agent of Contagious Porcine Pleuropneumonia, a disease responsible for economic losses in the swine industry worldwide. A. pleuropneumoniae is capable of producing proteinaceous exotoxins responsible for inducing hemorrhagic lesions, one of which is ApxI. Few studies have conducted an in-depth evaluation of polymorphisms of the nucleotides that make up the ApxI toxin gene. Here we analyze the polymorphisms of the apxIA gene region of A. pleuropneumoniae serovar 5 isolated from swine in different regions in Brazil and report the results of molecular sequencing and phylogenetic analysis. Analysis of the apxIA gene in 60 isolates revealed the presence of genetic diversity and variability. The polymorphisms in the nucleotide sequences determined the grouping of the Brazilian sequences and five more sequences from the GenBank database into 14 different haplotypes, which formed three main groups and revealed the presence of mutations in the nucleotide sequences. The estimation of selection pressures suggests the occurrence of genetic variations by positive selective pressure on A. pleuropneumoniae in large groups of animals in relatively small spaces. These conditions presumably favor the horizontal dissemination of apxIA gene mutations within bacterial populations with host reservoirs. As a result, the same serovar can demonstrate different antigenic capacities due to mutations in the apxIA gene. These alterations in sequences of the apxIA gene could occur in other areas of countries with intense swine production, which could lead to differences in the pathogenicity and immunogenicity of each serovar and have implications for the clinical status or diagnosis of A. pleuropneumoniae.

Treatment of infected lungs by ex vivo perfusion with high dose antibiotics and autotransplantation: A pilot study in pigs

The emergence of multi-drug resistant bacteria threatens to end the era of antibiotics. Drug resistant bacteria have evolved mechanisms to overcome antibiotics at therapeutic doses and further dose increases are not possible due to systemic toxicity. Here we present a pilot study of ex vivo lung perfusion (EVLP) with high dose antibiotic therapy followed by autotransplantation as a new therapy of last resort for otherwise incurable multidrug resistant lung infections. Severe Pseudomonas aeruginosa pneumonia was induced in the lower left lungs (LLL) of 18 Mini-Lewe pigs. Animals in the control group (n = 6) did not receive colistin. Animals in the conventional treatment group (n = 6) received intravenous application of 2 mg/kg body weight colistin daily. Animals in the EVLP group (n = 6) had their LLL explanted and perfused ex vivo with a perfusion solution containing 200 μg/ml colistin. After two hours of ex vivo treatment, autotransplantation of the LLL was performed. All animals were followed for 4 days following the initiation of treatment. In the control and conventional treatment groups, the infection-related mortality rate after five days was 66.7%. In the EVLP group, there was one infection-related mortality and one procedure-related mortality, for an overall mortality rate of 33.3%. Moreover, the clinical symptoms of infection were less severe in the EVLP group than the other groups. Ex vivo lung perfusion with very high dose antibiotics presents a new therapeutic option of last resort for otherwise incurable multidrug resistant pneumonia without toxic side effects on other organs.

Update on Actinobacillus pleuropneumoniae-knowledge, gaps and challenges

Porcine pleuropneumonia, caused by the bacterial porcine respiratory tract pathogen Actinobacillus pleuropneumoniae, leads to high economic losses in affected swine herds in most countries of the world. Pigs affected by peracute and acute disease suffer from severe respiratory distress with high lethality. The agent was first described in 1957 and, since then, knowledge about the pathogen itself, and its interactions with the host, has increased continuously. This is, in part, due to the fact that experimental infections can be studied in the natural host. However, the fact that most commercial pigs are colonized by this pathogen has hampered the applicability of knowledge gained under experimental conditions. In addition, several factors are involved in development of disease, and these have often been studied individually. In a DISCONTOOLS initiative, members from science, industry and clinics exchanged their expertise and empirical observations and identified the major gaps in knowledge. This review sums up published results and expert opinions, within the fields of pathogenesis, epidemiology, transmission, immune response to infection, as well as the main means of prevention, detection and control. The gaps that still remain to be filled are highlighted, and present as well as future challenges in the control of this disease are addressed.

Attenuated Actinobacillus pleuropneumoniae double-deletion mutant S-8∆clpP/apxIIC confers protection against homologous or heterologous strain challenge

Background

Actinobacillus pleuropneumoniae is the etiological agent of porcine pleuropneumonia, which leads to large economic losses to the swine industry worldwide. In this study, S-8△clpP△apxIIC, a double-deletion mutant of A. pleuropneumoniae was constructed, and its safety and protective efficacy were evaluated in pigs.

Results

The S-8△clpP△apxIIC mutant exhibited attenuated virulence in a murine (BALB/c) model, and caused no detrimental effects on pigs even at a dose of up to 1.0 × 10⁹ CFU. Furthermore, the S-8△clpP△apxIIC mutant was able to induce a strong immune response in pigs, which included high levels of IgG1 and IgG2, stimulated gamma interferon (IFN-γ), interleukin 12 (IL-12), and interleukin 4 (IL-4) production, and conferred effective protection against the lethal challenge with A. pleuropneumoniae serovars 7 or 5a. The pigs in the S-8△clpP△apxIIC immunized groups have no lesions and reduced bacterial loads in the lung tissue after challenge.

Conclusions

The data obtained in this study suggest that the S-8△clpP△apxIIC mutant can serve as a highly immunogenic and potential live attenuated vaccine candidate against A. pleuropneumoniae infection.

Immunoprotective Efficacy of Six In vivo-Induced Antigens against Actinobacillus pleuropneumoniae as Potential Vaccine Candidates in Murine Model

Six in vivo-induced (IVI) antigens—RnhB, GalU, GalT, Apl_1061, Apl_1166, and HflX were selected for a vaccine trial in a mouse model. The results showed that the IgG levels in each immune group was significantly higher than that of the negative control (P < 0.001). Except rRnhB group, proliferation of splenocytes was observed in all immunized groups and a relatively higher proliferation activity was observed in rGalU and rGalT groups (P < 0.05). In the rGalT vaccinated group, the proportion of CD4+ T cells in spleen was significant higher than that of negative control (P < 0.05). Moreover, proportions of CD4+ T cells in other vaccinated groups were all up-regulated to varying degrees. Up-regulation of both Th1 (IFN-γ, IL-2) and Th2 (IL-4) cytokines were detected. A survival rate of 87.5, 62.5, and 62.5% were obtained among rGalT, rAPL_1166, and rHflX group, respectively while the remaining three groups was only 25%. Histopathological analyses of lungs indicated that surviving animals from the vaccinated groups showed relatively normal pulmonary structure alveoli. These findings confirm that IVI antigens used as vaccine candidates provide partial protection against Actinobacillus pleuropneumoniae infection in a mouse model, which could be used as potential vaccine candidates in piglets.

The generation of successive unmarked mutations and chromosomal insertion of heterologous genes in Actinobacillus pleuropneumoniae using natural transformation

We have developed a simple method of generating scarless, unmarked mutations in Actinobacillus pleuropneumoniae by exploiting the ability of this bacterium to undergo natural transformation, and with no need to introduce plasmids encoding recombinases or resolvases. This method involves two successive rounds of natural transformation using linear DNA: the first introduces a cassette carrying cat (which allows selection by chloramphenicol) and sacB (which allows counter-selection using sucrose) flanked by sequences to either side of the target gene; the second transformation utilises the flanking sequences ligated directly to each other in order to remove the cat-sacB cassette. In order to ensure efficient uptake of the target DNA during transformation, A. pleuropneumoniae uptake sequences are added into the constructs used in both rounds of transformation. This method can be used to generate multiple successive deletions and can also be used to introduce targeted point mutations or insertions of heterologous genes into the A. pleuropneumoniae chromosome for development of live attenuated vaccine strains. So far, we have applied this method to highly transformable isolates of serovars 8 (MIDG2331), which is the most prevalent in the UK, and 15 (HS143). By screening clinical isolates of other serovars, it should be possible to identify other amenable strains.

Actinobacillus pleuropneumoniae challenge in swine: diagnostic of lung alterations by infrared thermography

BACKGROUND

Actinobacillus pleuropneumoniae (A.pp.) is the causative agent of porcine pleuropneumonia leading to high economic losses in the pig industry. Infrared thermography (IRT) of the thorax might offer a new method to select swine with lung alterations for further diagnostics. In this study 50 german landrace pigs were infected with A.pp. in an established model for respiratory tract disease, while 10 healthy pigs served as control animals. To avoid drift errors during IR measurements absolute skin temperatures and temperature differences between a thoracal and an abdominal region were assessed for its diagnostic validity.

RESULTS

IRT findings during the course of experimental A.pp.-infection were verified by computed tomography (CT) before and on days 4 and 21 after infection. Significant correlations were found between clinical scores, CT score and lung lesion score. Ambient temperature, body temperature and abdominal surface temperature were factors influencing the skin surface temperature of the thorax. On day 4 but not on day 21 after infection the right thoracal temperature was significantly higher and the difference between a thoracal region in the height of the left 10th vertebra and an abdominal region was significantly lower in infected pigs than in control pigs. At a cut off of 28°C of right thoracal temperature the specificity of the method was 100% (CI 95%: 69-100%) and the sensitivity 66% (CI 95%: 51-79%). At a cut off of 2°C temperature difference between thoracal and abdominal region on the left body site the specificity of the method was 100% (CI 95%: 69-100%) and the sensitivity 32% (CI 95%: 19-47%) with all control pigs detected negative. Orientation for lung biopsy by IRT resulted in 100% specificity and sensitivity (CI 95%: 69-100%) of bacteriological examination of tissue samples during the acute stage of infection.

CONCLUSION

IRT might be a valuable tool for the detection of inflammatory lung alterations in pigs, especially during the acute stage of infection and if ambient temperatures are constant during individual measurements. External and internal factors interfere with this method, so that its application in the field might be restricted to a selection of pigs for further diagnostic with adequate specificity.

Virus-like particles: promising platforms with characteristics of DIVA for veterinary vaccine design

In general, it is difficult to differentiate infected from vaccinated animals through vaccination with conventional vaccines, thereby impeding the serological surveillance of animal diseases. DIVA (differentiating infected from vaccinated animals) vaccine, originally known as marker vaccine, usually based on the absence of at least one immunogenic protein in the vaccine strain, allows DIVA in conjunction with a diagnostic test that detects antibodies against the antigens lacking in the vaccine strain. Virus-like particles (VLPs), composed of one or more structural proteins but no genomes of native viruses, mimic the organization and conformation of authentic virions but have no ability to self-replicate in cells, potentially yielding safer vaccine candidates. Since VLPs containing either monovalent or multivalent antigen can be produced in compliance with the requirements for serological surveillance, the use of VLP-based vaccines plays a promising role in DIVA vaccination strategies against animal diseases. Here, we critically reviewed VLPs and companion diagnostics with properties of DIVA for veterinary vaccine design, and three different VLPs as promising platforms for DIVA vaccination strategies in animals.

Construction and immunogenicity of a ∆apxIC/ompP2 mutant of Actinobacillus pleuropneumoniae and Haemophilus parasuis

The apxIC genes of the Actinobacillus pleuropneumoniae serovar 5 (SC-1), encoding the ApxIactivating proteins, was deleted by a method involving sucrose counter-selection. In this study, a mutant strain of A. pleuropneumoniae (SC-1) was constructed and named DapxIC/ ompP2. The mutant strain contained foreign DNA in the deletion site of ompP2 gene of Haemophilus parasuis. It showed no haemolytic activity and lower virulence of cytotoxicity in mice compared with the parent strain, and its safety and immunogenicity were also evaluated in mice. The LD50 data shown that the mutant strain was attenuated 30-fold, compared with the parent strain (LD50 of the mutant strain and parent strain in mice were determined to be 1.0 × 107 CFU and 3.5 × 105 CFU respectively). The mutant strain that was attenuated could secrete inactivated ApxIA RTX toxins with complete antigenicity and could be used as a candidate live vaccine strain against infections of A. pleuropneumoniae and H. parasuis.

PR-39, a porcine host defence peptide, is prominent in mucosa and lymphatic tissue of the respiratory tract in healthy pigs and pigs infected with Actinobacillus pleuropneumoniae

BACKGROUND

Host defence peptides are important components of mammalian innate immunity. We have previously shown that PR-39, a cathelicidin host defence peptide, is an important factor in porcine innate immune mechanisms as a first line of defence after infection with Actinobacillus pleuropneumoniae. PR-39 interacts with bacterial and mammalian cells and is involved in a variety of processes such as killing of bacteria and promotion of wound repair. In bronchoalveolar lavage fluid of infected pigs PR-39 concentrations are elevated during the chronic but not during the acute stage of infection when polymorphonuclear neutrophils (known as the major source of PR-39) are highly increased. Thus it was assumed, that the real impact of PR-39 during infection might not be reflected by its concentration in bronchoalveolar lavage fluid.

RESULTS

Using immunohistochemistry this study demonstrates the actual distribution of PR-39 in tissue of the upper and lower respiratory tract of healthy pigs, and of pigs during the acute and chronic stage of experimental infection with Actinobacillus pleuropneumoniae.During the acute stage of infection PR-39 accumulated adjacent to blood vessels and within bronchi. Immune reactions were mainly localized in the cytoplasm of cells with morphological characteristics of polymorphonuclear neutrophils as well as in extracellular fluids. During the chronic stage of infection pigs lacked clinical signs and lung alterations were characterized by reparation and remodelling processes such as tissue sequestration and fibroblastic pleuritis with a high-grade accumulation of small PR-39-positive cells resembling polymorphonuclear neutrophils. In healthy pigs, PR-39 was homogenously expressed in large single cells within the alveoli resembling alveolar macrophages or type 2 pneumocytes. PR-39 was found in all tissue samples of the upper respiratory tract in healthy and diseased pigs. Within the tracheobronchial lymph nodes, PR-39 dominated in the cytoplasm and nuclei of large cells resembling antigen-presenting cells located in the periphery of secondary follicles.

CONCLUSIONS

These immunohistochemical findings indicate that, in addition to polymorphonuclear neutrophils, other cells are involved in the expression, storage, or uptake of PR-39. The presence of PR-39 in healthy lung tissue showed that this antibacterial peptide might be important for the maintenance of health.

Experimental Actinobacillus pleuropneumoniae challenge in swine: comparison of computed tomographic and radiographic findings during disease

Background

In pigs, diseases of the respiratory tract like pleuropneumonia due to Actinobacillus pleuropneumoniae (App) infection have led to high economic losses for decades. Further research on disease pathogenesis, pathogen-host-interactions and new prophylactic and therapeutic approaches are needed. In most studies, a large number of experimental animals are required to assess lung alterations at different stages of the disease. In order to reduce the required number of animals but nevertheless gather information on the nature and extent of lung alterations in living pigs, a computed tomographic scoring system for quantifying gross pathological findings was developed. In this study, five healthy pigs served as control animals while 24 pigs were infected with App, the causative agent of pleuropneumonia in pigs, in an established model for respiratory tract disease.

Results

Computed tomographic (CT) findings during the course of App challenge were verified by radiological imaging, clinical, serological, gross pathology and histological examinations. Findings from clinical examinations and both CT and radiological imaging, were recorded on day 7 and day 21 after challenge. Clinical signs after experimental App challenge were indicative of acute to chronic disease. Lung CT findings of infected pigs comprised ground-glass opacities and consolidation. On day 7 and 21 the clinical scores significantly correlated with the scores of both imaging techniques. At day 21, significant correlations were found between clinical scores, CT scores and lung lesion scores. In 19 out of 22 challenged pigs the determined disease grades (not affected, slightly affected, moderately affected, severely affected) from CT and gross pathological examination were in accordance. Disease classification by radiography and gross pathology agreed in 11 out of 24 pigs.

Conclusions

High-resolution, high-contrast CT examination with no overlapping of organs is superior to radiography in the assessment of pneumonic lung lesions after App challenge. The new CT scoring system allows for quantification of gross pathological lung alterations in living pigs. However, computed tomographic findings are not informative of the etiology of respiratory disease.

A new variant of Actinobacillus pleuropneumoniae serotype 3 lacking the entire apxII operon

Actinobacillus pleuropneumoniae is an important respiratory pathogen causing pleuropneumonia in pig. The species is genetically characterized by the presence of 4 RTX (Repeats in the Structural ToXin) toxin genes: apxI, apxII, and apxIII genes are differentially present in various combinations among the different serotypes, thereby defining pathogenicity; the apxIV gene is present in all serotypes. Polymerase chain reaction (PCR)-based apx gene typing is done in many veterinary diagnostic laboratories, especially reference laboratories. The present report describes the isolation of atypical A. pleuropneumoniae from 4 independent cases from 2 countries. All isolates were beta-nicotinamide adenine dinucleotide (β-NAD) dependent and nonhemolytic but showed strong co-hemolysis with the sphingomyelinase of Staphylococcus aureus on sheep blood agar. Classical biochemical tests as well as Matrix-assisted laser desorption ionization time-of-flight mass spectrometry and sequence-based analysis (16S ribosomal RNA [rRNA] and rpoB genes) identified them as A. pleuropneumoniae. Apx-toxin gene typing using 2 different PCR systems showed the presence of apxIV and only the apxIII operon (apxIIICABD). None of the apxI or apxII genes were present as confirmed by Southern blot analysis. The 16S rRNA and rpoB gene analyses as well as serotype-specific PCR indicate that the isolates are variants of serotype 3. Strains harboring only apxIV and the apxIII operon are possibly emerging types of A. pleuropneumoniae and should therefore be carefully monitored for epidemiological reasons.

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.

Draft genome sequences of Actinobacillus pleuropneumoniae serotypes 2 and 6

Actinobacillus pleuropneumoniae is a bacterial pathogen that causes highly contagious respiratory infection in pigs and has a serious impact on the production economy and animal welfare. As clear differences in virulence between serotypes have been observed, the genetic basis should be investigated at the genomic level. Here, we present the draft genome sequences of the A. pleuropneumoniae serotypes 2 (strain 4226) and 6 (strain Femo).

Population-based analysis of Actinobacillus pleuropneumoniae ApxIVA for use as a DIVA antigen

APXIVA is an RTX toxin of Actinobacillus pleuropneumoniae that is a candidate antigen to differentiate infected from vaccinated animals (DIVA). Insertion of ISApl1 into the apxIVA gene is known to compromise an APXIVA-based DIVA approach, as is potentially a TGG to TGA mutation in the apxIVA gene. ISApl1 was found in 63/349 (18.1%) A. pleuropneumoniae isolates from England and Wales including serovars 2, 3, 6-8 and 12. No ISApl1 insertions into apxIVA were found. Only two serovar 3 isolates contained the TGG to TGA mutation. We conclude that an ApxIVA-based DIVA approach would potentially be viable in England and Wales.

A novel Respiratory Health Score (RHS) supports a role of acute lung damage and pig breed in the course of an Actinobacillus pleuropneumoniae infection

Background

Bacterial lung infections are a major cause of economic losses in the pig industry; they are responsible for approximately 50% of the antibiotics used in pigs and, therefore, also present an increasing concern to consumer protection agencies. In response to this changing market we investigated the feasibility of an old approach aimed at the breeding selection of more resistant pigs. As a first step in this direction we applied a new respiratory health score system to study the susceptibility of four different pig breeding lines (German Landrace, Piétrain, Hampshire, Large White) towards the respiratory tract pathogen Actinobacillus (A.) pleuropneumoniae.

Results

A controlled experimental aerosol infection with an A. pleuropneumoniae serotype 7 isolate was performed using 106 weaning pigs of defined breeding lines from the breeds German Landrace, Piétrain, Hamphire, and Large White. Pigs were clinically assessed on days 4 and 20 post infection following a novel scoring system, the Respiratory Health Score (RHS), which combines clinical, sonographic and radiographic examination results. The ranking on day 4 was significantly correlated with the ranking based on the pathomorphological Lung Lesion Score (LLS; Spearman Rank Correlation Coefficient of 0.86 [p < 0.0001]). Based on their RHS pigs were assigned to the different quartiles independent of the breeding line. The RHS-based rankings of pigs on day 4 and on day 20 were highly correlated (Spearman Rank Correlation Coefficient of 0.82 [p < 0.0001]) independent of the breeding line. Pigs of the Hampshire line were predominantly found in the lowest scoring quartile (47.6%) and absent in the highest scoring quartile. In contrast, pigs of the German Landrace and Piétrain breeding lines were predominantly found in the highest scoring quartile (32.3% and 35.7%, respectively).

Conclusion

These results demonstrate that the RHS obtained from live pigs shows a highly significant correlation to the lung lesion score considered as a "gold standard". The correlation of the ranking at days 4 and 20 post infection implies that the course of disease is highly dependent on the acute lung damage. The different severity of signs among the tested pig breeding lines clearly suggests a genetic difference in the susceptibility of pigs to A. pleuropneumoniae infection.

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.

Identification of immunogenic polypeptides from a Mycoplasma hyopneumoniae genome library by phage display

The identification of immunogenic polypeptides of pathogens is helpful for the development of diagnostic assays and therapeutic applications like vaccines. Routinely, these proteins are identified by two-dimensional polyacrylamide gel electrophoresis and Western blot using convalescent serum, followed by mass spectrometry. This technology, however, is limited, because low or differentially expressed proteins, e.g. dependent on pathogen-host interaction, cannot be identified. In this work, we developed and improved a M13 genomic phage display-based method for the selection of immunogenic polypeptides of Mycoplasma hyopneumoniae, a pathogen causing porcine enzootic pneumonia. The fragmented genome of M. hyopneumoniae was cloned into a phage display vector, and the genomic library was packaged using the helperphage Hyperphage to enrich open reading frames (ORFs). Afterwards, the phage display library was screened by panning using convalescent serum. The analysis of individual phage clones resulted in the identification of five genes encoding immunogenic proteins, only two of which had been previously identified and described as immunogenic. This M13 genomic phage display, directly combining ORF enrichment and the presentation of the corresponding polypeptide on the phage surface, complements proteome-based methods for the identification of immunogenic polypeptides and is particularly well suited for the use in mycoplasma species.

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.