Actinobacillus pleuropneumoniae infections in pigs: the role of virulence factors in pathogenesis and protection

Serotype diversity and antimicrobial susceptibility profiles of Actinobacillus pleuropneumoniae isolated in Italian pig farms from 2015 to 2022

Actinobacillus pleuropneumoniae (APP) is a bacterium frequently associated with porcine pleuropneumonia. The acute form of the disease is highly contagious and often fatal, resulting in significant economic losses for pig farmers. Serotype diversity and antimicrobial resistance (AMR) of APP strains circulating in north Italian farms from 2015 to 2022 were evaluated retrospectively to investigate APP epidemiology in the area. A total of 572 strains isolated from outbreaks occurring in 337 different swine farms were analysed. The majority of isolates belonged to serotypes 9/11 (39.2%) and 2 (28.1%) and serotype diversity increased during the study period, up to nine different serotypes isolated in 2022. The most common resistances were against tetracycline (53% of isolates) and ampicillin (33%), followed by enrofloxacin, florfenicol and trimethoprim/sulfamethoxazole (23% each). Multidrug resistance (MDR) was common, with a third of isolates showing resistance to more than three antimicrobial classes. Resistance to the different classes and MDR varied significantly depending on the serotype. In particular, the widespread serotype 9/11 was strongly associated with florfenicol and enrofloxacin resistance and showed the highest proportion of MDR isolates. Serotype 5, although less common, showed instead a concerning proportion of trimethoprim/sulfamethoxazole resistance. Our results highlight how the typing of circulating serotypes and the analysis of their antimicrobial susceptibility profile are crucial to effectively manage APP infection and improve antimicrobial stewardship.

Impact of bronchoalveolar lavage from influenza A virus diseased pigs on neutrophil functions and growth of co-infecting pathogenic bacteria

Introduction

Influenza A viruses (IAVs) infect the respiratory tract of mainly humans, poultry, and pigs. Co-infections with pathogenic lung bacteria are a common event and contribute to the severity of disease progression. Neutrophils are a major cell type of the innate immune system and are rapidly recruited to the site of infection. They have several effector functions to fight invading pathogens such as the secretion of reactive oxygen species (ROS) or the release of neutrophil extracellular traps (NETs). NETs are known to promote the growth of Pasteurellaceae bacteria, especially if degraded by nucleases.

Methods

In this study, bronchoalveolar lavage fluid (BALF) from 45 field-infected pigs was analyzed for 1) NET markers, 2) influence on growth of lung bacteria, and 3) impact on neutrophil functions. BALF samples from 21 IAV-positive pigs and 24 lung diseased but IAV-negative pigs were compared.

Results

Here, we show that neutrophils in the lungs of IAV-positive pigs release vesicular NETs. Several NET markers were increased in the BALF of IAV-positive pigs compared with the BALF from IAV-negative pigs. The amount of NET markers positively correlated with the viral load of the IAV infection. Interestingly, the BALF of IAV-positive pigs enhanced the growth of bacteria belonging to the family of Pasteurellaceae as potential coinfecting bacteria. These effects were weaker with the BALF derived from IAV-negative pigs with other lung infections. The intensity of oxidative burst in neutrophils was significantly decreased by BALF from IAVpositive pigs, indicating impaired antimicrobial activity of neutrophils. Finally, the lung milieu reflected by IAV-positive BALF does not enable neutrophils to kill Actinobacillus pleuropneumoniae but rather enhances its growth.

Discussion

In summary, our data show that an IAV infection is affecting neutrophil functions, in particular the release of NETs and ROS. Furthermore, IAV infection seems to provide growth-enhancing factors for especially coinfecting Pasteurellaceae and reduces the killing efficiency of neutrophils.

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.

TurboID screening of ApxI toxin interactants identifies host proteins involved in Actinobacillus pleuropneumoniae-induced apoptosis of immortalized porcine alveolar macrophages

Actinobacillus pleuropneumoniae (APP) is a gram-negative pathogenic bacterium responsible for porcine contagious pleuropneumonia (PCP), which can cause porcine necrotizing and hemorrhagic pleuropneumonia. Actinobacillus pleuropneumoniae-RTX-toxin (Apx) is an APP virulence factor. APP secretes a total of four Apx toxins, among which, ApxI demonstrates strong hemolytic activity and cytotoxicity, causing lysis of porcine erythrocytes and apoptosis of porcine alveolar macrophages. However, the protein interaction network between this toxin and host cells is still poorly understood. TurboID mediates the biotinylation of endogenous proteins, thereby targeting specific proteins and local proteomes through gene fusion. We applied the TurboID enzyme-catalyzed proximity tagging method to identify and study host proteins in immortalized porcine alveolar macrophage (iPAM) cells that interact with the exotoxin ApxI of APP. His-tagged TurboID-ApxIA and TurboID recombinant proteins were expressed and purified. By mass spectrometry, 318 unique interacting proteins were identified in the TurboID ApxIA-treated group. Among them, only one membrane protein, caveolin-1 (CAV1), was identified. A co-immunoprecipitation assay confirmed that CAV1 can interact with ApxIA. In addition, overexpression and RNA interference experiments revealed that CAV1 was involved in ApxI toxin-induced apoptosis of iPAM cells. This study provided first-hand information about the proteome of iPAM cells interacting with the ApxI toxin of APP through the TurboID proximity labeling system, and identified a new host membrane protein involved in this interaction. These results lay a theoretical foundation for the clinical treatment of PCP.

Adh Promotes Actinobacillus pleuropneumoniae Survival in Porcine Alveolar Macrophages by Inhibiting CHAC2-Mediated Respiratory Burst and Inflammatory Cytokine Expression

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) causes porcine pleuropneumonia that seriously endangers pig's health. Adh, located in the head region of trimeric autotransporter adhesion of A. pleuropneumoniae, affects bacterial adhesion and pathogenicity. However, how Adh mediates A. pleuropneumoniae immune invasion is still unclear. Here, we established the A. pleuropneumoniae strain L20 or L20 ΔAdh-infected porcine alveolar macrophages (PAM) model, and applied protein overexpression, RNA interference, qRT-PCR, Western blot and immunoflourescence techniques to dissect the effects of Adh on PAM during A. pleuropneumoniae infection. We found that Adh could increase the A. pleuropneumoniae adhesion and intracellular survival in PAM. Gene chip analysis of piglet lungs further showed that Adh significantly induced cation transport regulatory-like protein 2 (CHAC2) expression, whose overexpression suppressed the phagocytic capacity of PAM. Furthermore, CHAC2 overexpression dramatically increased glutathione (GSH) expression, decreased reactive oxygen species (ROS), and promoted A. pleuropneumoniae survival in PAM, while the knockdown of CHAC2 reversed these phenomena. Meanwhile, CHAC2 silence activated the NOD1/NF-κB pathway, resulting in an increase in IL-1β, IL-6, and TNF-α expression, whereas this effect was weakened by CHAC2 overexpression and addition of NOD1/NF-κB inhibitor ML130. Moreover, Adh enhanced the secretion of LPS of A. pleuropneumoniae, which regulated the expression of CHAC2 via TLR4. In conclusion, through a LPS-TLR4-CHAC2 pathway, Adh inhibits respiratory burst and inflammatory cytokines expression to promote A. pleuropneumoniae survival in PAM. This finding may provide a novel target for the prevention and treatment of A. pleuropneumoniae.

Proteomic and immunoproteomic insights into the exoproteome of Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia

Porcine pleuropneumonia caused by Actinobacillus pleuropneumoniae affects pig health status and the swine industry worldwide. Despite the extensive number of studies focused on A. pleuropneumoniae infection and vaccine development, a thorough analysis of the A. pleuropneumoniae exoproteome is still missing. Using a complementary approach of quantitative proteomics and immunoproteomics we gained an in-depth insight into the A. pleuropneumoniae serotype 2 exoproteome, which provides the basis for future functional studies. Label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) revealed 593 exoproteins, of which 104 were predicted to be virulence factors. The RTX toxins ApxIIA and ApxIIIA -were found to be the most abundant proteins in the A. pleuropneumoniae serotype 2 exoproteome. Furthermore, the ApxIVA toxin was one of the proteins showing the highest abundance, although ApxIVA is commonly assumed to be expressed exclusively in vivo. Our study revealed several antigens, including proteins with moonlight functions, such as the elongation factor (EF)-Tu, and proteins linked to specific metabolic traits, such as the maltodextrin-binding protein MalE, that warrant future functional characterization and might present potential targets for novel therapeutics and vaccines. Our Ig-classes specific serological proteome analysis (SERPA) approach allowed us to explore the development of the host humoral immune response over the course of the infection. These SERPAs pinpointed proteins that might play a key role in virulence and persistence and showed that the immune response to the different Apx toxins is distinct. For instance, our results indicate that the ApxIIIA toxin has properties of a thymus-independent antigen, which should be studied in more detail.

Hypersecretion of OmlA antigen in Corynebacterium glutamicum through high-throughput based development process

Outer membrane lipoprotein A (OmlA) is a vaccine antigen against porcine contagious pleuropneumonia (PCP), a disease severely affecting the swine industry. Here, we aimed to systematically potentiate the secretory production of OmlA in Corynebacterium glutamicum (C. glutamicum), a widely used microorganism in the food industry, by establishing a holistic development process based on our high-throughput culture platform. The expression patterns, expression element combinations, medium composition, and induction conditions were comprehensively screened or optimized in microwell plates (MWPs), followed by fermentation parameter optimization in a 4 × 1 L parallel fermentation system (CUBER4). An unprecedented yield of 1.01 g/L OmlA was ultimately achieved in a 5-L bioreactor following the scaling-up strategy of fixed oxygen mass transfer coefficient (k_La), and the produced OmlA antigen showed well-protective immunity against Actinobacillus pleuropneumoniae challenge. This result provides a rapid and reliable pipeline to achieve the hyper-production of OmlA, and possibly other recombinant vaccines, in C. glutamicum. KEY POINTS: • Established a holistic development process and applied it to potentiate the secretion of OmlA. • The secretion of OmlA reached an unprecedented yield of 1.01 g/L. • The recombinant OmlA antigen induced efficient protective immunity.

Pharmacokinetic/Pharmacodynamic Relationships of Tulathromycin Against Actinobacillus pleuropneumoniae in a Porcine Tissue Cage Infection Model

Tulathromycin is a semi-synthetic macrolide antibiotic that is highly effective in treating respiratory tract bacterial infections. We evaluated the in vivo antibacterial activity of tulathromycin against Actinobacillus pleuropneumoniae in piglets and determined its pharmacokinetic/pharmacodynamic (PK/PD) relationships using a tissue cage infection model. A. pleuropneumoniae (108 CFU/ml) was exposed to tulathromycin via intramuscular injection followed by a collection of cage tissue fluids at various intervals. The percentage of time the drug concentration remained above the minimum inhibitory concentration (MIC) divided by the dosing interval (%T > MIC) was the best PK/PD index to describe the antibacterial efficacy of tulathromycin (R 2 = 0.9421). The %T > MIC values required to achieve 1 - log10CFU/ml reductions and bactericidal activity (3 - log10CFU/ml reduction) were 50.8 and 96.38%, respectively. These results demonstrated that maintaining %T > MIC above 96.38% achieved bactericidal activity and thereby optimized the clinical dosage.

A novel Dicistro-like virus discovered in Procambarus clarkii with "Black May" disease

The peak period of morbidity and death in cultured Procambarus clarkii is around May each year and is called the "Black May" disease. The pathogen causing "Black May" disease is believed to be a white spot syndrome virus (WSSV). In 2018, a significant number of P. clarkii died in the pond culture of Xinglong Township, Xuyi County. Two sampling tests on the affected pond showed that, in addition to WSSV, a novel Dicistro-like virus (PcDV) was present. Genomic sequence analysis indicated that this new virus belongs to the Dicistroviridae family, Picornaviridaes order. A high number of spherical particles were detected in gill tissues of P. clarkii with "Black May" disease by electron microscopy, a finding consistent with viruses from the Picornaviridaes order. From October 2018 to September 2019, we took monthly samples from Hubei, Jiangsu and Anhui provinces, and tested for the presence of PcDV and WSSV in P. clarkii. The detection rates of PcDV in P. clarkii peaked from April to June, consistent with the onset of the "Black May" disease. In conclusion, we believe that the discovery of PcDV will provide new research directions for investigating the pathogens causing "Black May" disease in P. clarkii.

Molecular and functional characterization of HtrA protein in Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae (A.pleuropneumoniae) causes serious economic loss for the swine industry. A high-temperature requirements A (HtrA)-like protease and its homologs have been reported to be involved in protein quality control and expression of important immunoprotective antigens in many pathogens. In this study, we showed that HtrA of A.pleuropneumoniae exhibited both chaperone and proteolytic activities. Moreover, Outer membrane protein P5 (OmpP5) in A.pleuropneumoniae and Heat shock protein 90 (Hsp90) in porcine lung tissues were first discovered and identified as specific proteolytic substrates for rHtrA. The maximum cleavage activity occurs at 50 ℃ in a time-dependent manner. In addition, rHtrA mainly induced IgG 2a subtype of IgG and Th1 (IFN-γ, IL-2) response in a mice model, and promoted a significant proliferation of spleen lymphocytes compare with negative control (P < 0.05). The survival rates of 37.5 % were observed against A.pleuropneumoniae strain. Together, these data demonstrate that rHtrA plays a multi-functional role in A.pleuropneumoniae.

A Multivalent Vaccine Containing Actinobacillus pleuropneumoniae and Mycoplasma hyopneumoniae Antigens Elicits Strong Immune Responses and Promising Protection in Pigs

Actinobacillus pleuropneumoniae (App) and Mycoplasma hyopneumoniae (Mhp) cause porcine pleuropneumonia and mycoplasmal pneumonia, respectively, and have serious impacts on the swine industry because they retard the growth of pigs. To protect pigs against these diseases, we have developed a multivalent vaccine consisting of App bacterins, APP RTX toxins (Apx toxins), and Mhp bacterin and adhesin protein. This vaccine induced the production of higher levels of antibodies against App and Mhp than the commercial vaccine (Nisseiken Swine APM Inactivated Vaccine). Furthermore, the vaccine efficiently protected pigs against virulent App challenge, showing promise as an efficient vaccine for the prevention of two important respiratory diseases, porcine pleuropneumonia and mycoplasmal pneumonia.

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.

Genomic Analysis of IgG Antibody Response to Common Pathogens in Commercial Sows in Health-Challenged Herds

Losses due to infectious diseases are one of the main factors affecting productivity in the swine industry, motivating the investigation of disease resilience-related traits for genetic selection. However, these traits are not expected to be expressed in the nucleus herds, where selection is performed. One alternative is to use information from the commercial level to identify and select nucleus animals genetically superior for coping with pathogen challenges. In this study, we analyzed the genetic basis of antibody (Ab) response to common infectious pathogens in health-challenged commercial swine herds as potential indicator traits for disease resilience, including Ab response to influenza A virus of swine (IAV), Mycoplasma hyopneumoniae (MH), porcine circovirus (PCV2), and Actinobacillus pleuropneumoniae (APP; different serotypes). Ab response was measured in blood at entry into gilt rearing, post-acclimation (∼40 days after entering the commercial herd), and parities 1 and 2. Heritability estimates for Ab response to IAV, MH, and PCV2 ranged from 0 to 0.76. Ab response to APP ranged from 0 to 0.40. The genetic correlation (r _G ) of Ab response to IAV with MH, PCV2, PRRSV, and APP_mean (average Ab responses for all serotypes of APP) were positive (>0.29) at entry. APP_mean was negatively correlated with PCV2 and MH at entry and parity 2 but positively correlated with MH at post-acclimation and parity 1. Genomic regions associated with Ab response to different APP serotypes were identified on 13 chromosomes. The region on chromosome 14 (2 Mb) was associated with several serotypes of APP, explaining up to 4.3% of the genetic variance of Ab to APP7 at entry. In general, genomic prediction accuracies for Ab response were low to moderate, except average Ab response to all infectious pathogens evaluated. These results suggest that genetic selection of Ab response in commercial sows is possible, but with variable success depending on the trait and the time-point of collection. Future work is needed to determine genetic correlations of Ab response with disease resilience, reproductive performance, and other production traits.

Efficacy of enteric-coated tilmicosin granules in pigs artificially infected with Actinobacillus pleuropneumoniae serotype 2

BACKGROUND

Porcine infectious pleuropneumonia caused by Actinobacillus pleuropneumoniae (App) is one of the most serious infectious diseases in pigs and has brought huge economic losses to the world pig industry. The aim of this trial was to evaluate the effect of enteric-coated tilmicosin granule in the treatment and control of artificial infection of App.

METHODS

Sixty Duroc and Yorkshire crossbred pigs (50 of which were artificially infected) were divided into six groups: BCG (Blank control group), ICG (Infection-only control group), HDG (High-dose enteric-coated tilmicosin granules), MDG (Medium-dose enteric-coated tilmicosin granules), LDG (Low-dose enteric-coated tilmicosin granules) and TPG (Tilmicosin premix drug control group). The cure rate, mortality, clinical respiratory score, body temperature score, weight gain, lung score and so on were recorded.

RESULTS

The cure rate of HDG and MDG was as high as 90%, the mortality was 10%, and the clinical signs recovered quickly.

CONCLUSION

The results showed that enteric-coated tilmicosin granules had obvious therapeutic effect on artificial infection, which could reduce the damage caused by the disease and reduce the mortality.

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.

Global Gene Networks in 3D4/31 Porcine Alveolar Macrophages Treated with Antigenic Epitopes of Actinobacillus pleuropneumoniae ApxIA, IIA, and IVA

Actinobacillus pleuropneumoniae (App) is the causative agent of porcine pleuropneumonia. Although App produces several virulence factors, Apx toxins, the primary App virulence factors, have been the focus of numerous studies. However, the host response against the Apx toxins has not been elucidated at the transcriptomic level. Therefore, in this study, we examined the response of an immortalized porcine alveolar macrophage cell line (IPAM 3D4/31) to four antigenic epitopes of the App exotoxins, ApxIA, IIA and IVA. The antigenic epitopes of the Apx toxins (ApxIA Ct, ApxIIA Nt, ApxIVA C1 and ApxIV C2) were determined by an in-silico antigenicity prediction analysis. Gene expression in IPAMs was analyzed by RNA-Seq after treatment with the four proteins for 24 h. A total of 15,269 DEGs were observed to be associated with cellular and metabolic processes in the GO category Biological Process and nuclear receptors and apoptosis signaling in IPA analyses. These DEGs were also related to M2 macrophage polarization and apoptosis in IPAMs. These host transcriptional analyses present novel global gene networks of the host response to treatment with Apx toxins.

Multi-organ spreading of Actinobacillus pleuropneumoniae serovar 7 in weaned pigs during the first week after experimental infection

Actinobacillus (A.) pleuropneumoniae is normally considered strictly adapted to the respiratory tract of swine. Despite this, scattered case reports of arthritis, osteomyelitis, hepatitis, meningitis or nephritis exist, in which A. pleuropneumoniae remained the only detectable pathogen. Therefore, the aim of this study was to investigate whether spreading to other organs than the lungs is incidental or may occur more frequently. For this, organ samples (blood, liver, spleen, kidney, tarsal and carpal joints, meninges, pleural and pericardial fluids) from weaners (n = 47) infected experimentally with A. pleuropneumoniae serovar 7 by aerosol infection (infection dose: 10.9 × 10³ cfu/animal) were examined by culture during the first week after infection. In addition, tissue samples of eight weaners were examined by histology and immunohistochemistry (IHC). A. pleuropneumoniae was isolated in all examined sample sites (86.7% pleural fluids, 73.3% pericardial fluids, 50.0% blood, 61.7% liver, 51.1% spleen, 55.3% kidney, 14.9% tarsal joints, 12.8% carpal joints, 27.7% meninges). These results were also obtained from animals with only mild clinical symptoms. IHC detection confirmed these findings in all locations except carpal joints. Histological examination revealed purulent hepatitis (n = 2), nephritis (n = 1) and beginning meningitis (n = 2). Isolation results were significantly correlated (p < 0.001) with the degree of lung colonization and, to a lower extent, with the severity of disease. Detection of A. pleuropneumoniae in peripheral tissues was significantly correlated to spleen colonization. In conclusion, multi-organ spreading of A. pleuropneumoniae serovar 7 strain AP 76 seems to occur more frequently during acute infection following effective lung colonization than previously thought.

A trivalent Apx-fusion protein delivered by E. coli outer membrane vesicles induce protection against Actinobacillus pleuropneumoniae of serotype 1 and 7 challenge in a murine model

Actinobacillus pleuropneumoniae (APP) causes serious economic losses in the swine industry, and is the etiologic agent of porcine pleuropneumonia. In this study we have engineered a trivalent Apx fusion protein enclosed in outer membrane vesicles (Apxr-OMV) and studied its immunoprotective efficacy against APP serotypes 1 and 7 challenge in mice. The results showed that the IgG levels in the Apxr-OMVs immune group were significantly higher than those of the negative control (P < 0.05). Up-regulation of both Th1 (IFN-γ, IL-2) and Th2 (IL-4) cytokines were detected in splenocytes of Apxr-OMVs immune group. The survival rates 87.5% and 62.5% were observed against APP strain 1516 of serotype 7 and APP strain 2701 of serotype 1 in the groups of Apxr-OMVs immune group, respectively. Histopathological lesions of the pulmonary structure alveoli were found to be minimal in APX-OMV group challenged with APP serotypes 1 and 7. These results strongly indicated that engineered OMVs could effectively induce specific humoral or cellular immune responses. Moreover, Apxr-OMVs used as novel vaccine provides cross-protective immunity against different serotype 1 and 7 of APP infection in a mouse model. In contrast, the OMV-empty and PBS as negative controls or inactivated strain of APP-2701 and APP-1516 as positive controls for the animal study cannot provide protection or cross-protection.

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.

Involvement of NF-κB in regulation of Actinobacillus pleuropneumoniae exotoxin ApxI-induced proinflammatory cytokine production in porcine alveolar macrophages

Actinobacillus pleuropneumoniae is a crucial respiratory pathogen that causes fibrinous, hemorrhagic, necrotizing pleuropneumonia in pigs. A. pleuropneumoniae exotoxins (ApxI to IV) are the major virulence factors contributing to A. pleuropneumoniae pathogenesis. Previously, we demonstrated that ApxI induces the expression of proinflammatory cytokines in porcine alveolar macrophages (PAMs) via the mitogen-activated protein kinases (MAPKs) p38 and cJun NH2-terminal kinase (JNK). Nonetheless, the role of nuclear factor (NF)-κB-a transcription factor widely implicated in immune and inflammatory responses-in ApxI-elicited cytokine production has yet to be defined. In the present study, we examined the involvement of NF-κB in ApxI-elicited production of interleukin (IL)-1β, IL-8, and tumor necrosis factor (TNF)-α in PAMs and investigated the correlation between NF-κB and MAPK (p38 and JNK) pathways in this event. The results of Western blot analysis, confocal microscopy, and a DNA binding activity assay revealed that the classical NF-κB pathway was activated by ApxI, as evidenced by the decreased levels of IκB and subsequent NF-κB translocation and activation in ApxI-stimulated PAMs. Moreover, the blocking of ApxI-induced NF-κB activation significantly attenuated the levels of mRNA and protein secretion of IL-1β, IL-8, and TNF-α in PAMs. Notably, the attenuation of JNK activation by a specific inhibitor (SP600125) reduced ApxI-induced NF-κB activation, whereas a p38 blocker (SB203580) had no effect on the NF-κB pathway. Further examination revealed that the level of phosphorylation at serine 536 on the NF-κB p65 subunit was dependent on JNK activity. Collectively, this study, for the first time, demonstrates a pivotal role of NF-κB in ApxI-induced IL-1β, IL-8, and TNF-α production; JNK, but not p38, may positively affect the activation of the classical NF-κB pathway.

Impact of Actinobacillus pleuropneumoniae biofilm mode of growth on the lipid A structures and stimulation of immune cells

Actinobacillus pleuropneumoniae (APP), the etiologic agent of porcine pleuropneumonia, forms biofilms on biotic and abiotic surfaces. APP biofilms confers resistance to antibiotics. To our knowledge, no studies have examined the role of APP biofilm in immune evasion and infection persistence. This study was undertaken to (i) investigate biofilm-associated LPS modifications occurring during the switch to biofilm mode of growth; and (ii) characterize pro-inflammatory cytokines expression in porcine pulmonary alveolar macrophages (PAMs) and proliferation in porcine PBMCs challenged with planktonic or biofilm APP cells. Extracted lipid A samples from biofilm and planktonic cultures were analyzed by HPLC high-resolution, accurate mass spectrometry. Biofilm cells displayed significant changes in lipid A profiles when compared with their planktonic counterparts. Furthermore, in vitro experiments were conducted to examine the inflammatory response of PAMs exposed to UV-inactivated APP grown in biofilm or in suspension. Relative mRNA expression of pro-inflammatory genes IL1, IL6, IL8 and MCP1 decreased in PAMs when exposed to biofilm cells compared to planktonic cells. Additionally, the biofilm state reduced PBMCs proliferation. Taken together, APP biofilm cells show a weaker ability to stimulate innate immune cells, which could be due, in part, to lipid A structure modifications.

Production and immunogenicity of Actinobacillus pleuropneumoniae ApxIIA protein in transgenic rice callus

Actinobacillus pleuropneumoniae is a major etiological agent that is responsible for swine pleuropneumonia, a highly contagious respiratory infection that causes severe economic losses in the swine production industry. ApxIIA is one of the virulence factors in A. pleuropneumoniae and has been considered as a candidate for developing a vaccine against the bacterial infection. A gene encoding an ApxIIA fragment (amino acids 439-801) was modified based on a plant-optimized codon and constructed into a plant expression vector under the control of a promoter and the 3' UTR of the rice amylase 3D gene. The plant expression vector was introduced into rice embryogenic callus (Oryza sativa L. cv. Dongjin) via particle bombardment-mediated transformation. The integration and transcription of the ApxIIA_439-801 gene were confirmed by using genomic DNA PCR amplification and Northern blot analysis, respectively. The synthesis of ApxIIA_439-801 antigen protein in transgenic rice callus was confirmed by western blot analysis. The concentration of antigen protein in lyophilized samples of transgenic rice callus was 250 μg/g. Immunizing mice with protein extracts from transgenic plants intranasally elicited secretory IgA. These results demonstrate the feasibility of using a transgenic plant to elicit immune responses against A. pleuropneumoniae.

NMR assignment of intrinsically disordered self-processing module of the FrpC protein of Neisseria meningitidis

The self-processing module (SPM) is an internal segment of the FrpC protein (P415-F591) secreted by the pathogenic Gram-negative bacterium Neisseria meningitidis during meningococcal infection of human upper respiratory tract. SPM mediates 'protein trans-splicing', a unique natural mechanism for editing of proteins, which involves a calcium-dependent autocatalytic cleavage of the peptide bond between D414 and P415 and covalent linkage of the cleaved fragment through its carboxy-terminal group of D414 to [Formula: see text]-amino group of lysine residue within a neighboring polypeptide chain. We present an NMR resonance assignment of the calcium-free SPM, which displays characteristic features of intrinsically disordered proteins. Non-uniformly sampled 5D HN(CA)CONH, 4D HCBCACON, and HCBCANCO spectra were recorded to resolve poorly dispersed resonance frequencies of the disordered protein and 91 % of SPM residues were unambiguously assigned. Analysis of the chemical shifts revealed that two regions of the intrinsically disordered SPM (A95-S101 and R120-I127) have a tendency to form a helical structure, whereas the residues P1-D7 and G36-A40 have the propensity to adopt a [Formula: see text]-structure.

Concurrent host-pathogen gene expression in the lungs of pigs challenged with Actinobacillus pleuropneumoniae

Background

Actinobacillus pleuropneumoniae causes pleuropneumonia in pigs, a disease which is associated with high morbidity and mortality, as well as impaired animal welfare. To obtain in-depth understanding of this infection, the interplay between virulence factors of the pathogen and defense mechanisms of the porcine host needs to be elucidated. However, research has traditionally focused on either bacteriology or immunology; an unbiased picture of the transcriptional responses can be obtained by investigating both organisms in the same biological sample.

Results

Host and pathogen responses in pigs experimentally infected with A. pleuropneumoniae were analyzed by high-throughput RT-qPCR. This approach allowed concurrent analysis of selected genes encoding proteins known or hypothesized to be important in the acute phase of this infection. The expression of 17 bacterial and 31 porcine genes was quantified in lung samples obtained within the first 48 hours of infection. This provided novel insight into the early time course of bacterial genes involved in synthesis of pathogen-associated molecular patterns (lipopolysaccharide, peptidoglycan, lipoprotein) and genes involved in pattern recognition (TLR4, CD14, MD2, LBP, MYD88) in response to A. pleuropneumoniae. Significant up-regulation of proinflammatory cytokines such as IL1B, IL6, and IL8 was observed, correlating with protein levels, infection status and histopathological findings. Host genes encoding proteins involved in iron metabolism, as well as bacterial genes encoding exotoxins, proteins involved in adhesion, and iron acquisition were found to be differentially expressed according to disease progression. By applying laser capture microdissection, porcine expression of selected genes could be confirmed in the immediate surroundings of the invading pathogen.

Conclusions

Microbial pathogenesis is the product of interactions between host and pathogen. Our results demonstrate the applicability of high-throughput RT-qPCR for the elucidation of dual-organism gene expression analysis during infection. We showed differential expression of 12 bacterial and 24 porcine genes during infection and significant correlation of porcine and bacterial gene expression. This is the first study investigating the concurrent transcriptional response of both bacteria and host at the site of infection during porcine respiratory infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1557-6) contains supplementary material, which is available to authorized users.

Efficacy of vaccination against Actinobacillus pleuropneumoniae in two Belgian farrow-to-finish pig herds with a history of chronic pleurisy

The efficacy of an Actinobacillus pleuropneumoniae subunit vaccine based on ApxIA, ApxIIA, ApxIIIA and OMP-2 (Porcilis App, MSD) was investigated in two farrow-to-finish pig herds (A and B) affected by chronic pleurisy. In total, 1161 pigs were included. At three weeks of age, the pigs were randomly allocated to non-vaccinated control (NV; n=580) and vaccinated (V; n=581) groups. At 6 and 10 weeks of age, pigs were injected with Porcilis-APP (V group) or adjuvant (NV group). At slaughter (26 weeks), pleurisy and pneumonia lesions were assessed. All pigs were weighed individually at 6 and 26 weeks of age, and average daily weight gain (ADG; g/pig/day) was calculated. Mortality and days of additional treatment (DAT) were registered during the whole experiment. Data were analysed using binary logistic regression or analysis of variance for proportions or continuous variables, respectively. The prevalence of pleurisy and pneumonia was (NV-A=19.3, V-A=7.9, (P=0.000); NV-B=17.9, V-B=0.7, (P=0.000)) and (NV-A=42.4, V-A=21.2, (P=0.000); NV-B=46.7, V-B=19.0, (P=0.000)), respectively. The ADG was NV-A=632±157, V-A=647±91, (P=0.162); NV-B=660±115, V-B=670±82, (P=0.232). The mortality during the experiment was NV-A=5.7, V-A=1.8, (P=0.015); NV-B=2.3, V-B=1.0, (P=0.170) per cent. The DAT was: NV-A=15.04±1.41, V-A=14.95±0.67, (P=0.010); NV-B=21.68±2.43, V-B=16.99±0.62, (P=0.000). The present study showed a significant reduction of the prevalence of pleurisy and pneumonia, and antimicrobial use in V pigs from both herds, and in mortality in V pigs from one herd.

A recombinant chimera comprising the R1 and R2 repeat regions of M. hyopneumoniae P97 and the N-terminal region of A. pleuropneumoniae ApxIII elicits immune responses

Background

Infection by Mycoplasma hyopneumoniae and Actinobacillus pleuropneumoniae, either alone or together, causes serious respiratory diseases in pigs.

Results

To develop an efficient multi-disease subunit vaccine against these pathogens, we produced a chimeric protein called Ap97, which comprises a deletion derivative of the N-terminal region of the A. pleuropneumoniae ApxIII toxin (ApxN) and the R1 and R2 repeats of M. hyopneumoniae P97 adhesin (P97C), using an E. coli expression system.

The levels of both IgG1 and IgG2a isotypes specific for ApxN and P97C in the sera of Ap97-immunized mice increased, and Ap97 induced the secretion of IL-4 and IFN-γ by mouse splenocytes. Antisera from mice and pigs immunized with Ap97 readily reacted with both native ApxIII and P97 proteins. In addition, immunization with the Ap97 vaccine effectively protected pigs against challenge with both pathogens.

Conclusions

These findings suggest that Ap97 confers immunogenicity, and is an effective vaccine that protects pigs against infection by M. hyopneumoniae and A. pleuropneumoniae.

Oral immunization of mice with Saccharomyces cerevisiae expressing a neutralizing epitope of ApxIIA exotoxin from Actinobacillus pleuropneumoniae induces systemic and mucosal immune responses

An oral delivery system based on ApxIIA#5-expressed on Saccharomyces cerevisiae was studied for its potential to induce immune responses in mice. Murine bone marrow-derived dendritic cells (DCs) stimulated in vitro with ApxIIA#5-expressed on S. cerevisiae upregulated the expression of maturation and activation markers, leading to production of tumor necrosis factor-α, interleukin (IL)-1β, IL-12p70 and IL-10. Presentation of these activated DCs to cluster of differentiation CD4+ T cells collected from mice that had been orally immunized with the ApxIIA#5-expressed on S. cerevisiae elicited specific T-cell proliferation. In addition, the orally immunized mice had stronger antigen-specific serum IgG and IgA antibody responses and larger numbers of antigen-specific IgG and IgA antibody-secreting cells in their spleens, Peyer's patches and lamina propria than did those immunized with vector-only S. cerevisiae or those not immunized. Furthermore, oral immunization induced T helper 1-type immune responses mediated via increased serum concentrations of IgG2a and an increase predominantly of IFN-γ-producing cells in their spleens and lamina propria. Our findings suggest that surface-displayed ApxIIA#5-expressed on S. cerevisiae may be a promising candidate for an oral vaccine delivery system for eliciting systemic and mucosal immunity.

Experimental identification of Actinobacillus pleuropneumoniae strains L20 and JL03 heptosyltransferases, evidence for a new heptosyltransferase signature sequence

We experimentally identified the activities of six predicted heptosyltransferases in Actinobacillus pleuropneumoniae genome serotype 5b strain L20 and serotype 3 strain JL03. The initial identification was based on a bioinformatic analysis of the amino acid similarity between these putative heptosyltrasferases with others of known function from enteric bacteria and Aeromonas. The putative functions of all the Actinobacillus pleuropneumoniae heptosyltrasferases were determined by using surrogate LPS acceptor molecules from well-defined A. hydrophyla AH-3 and A. salmonicida A450 mutants. Our results show that heptosyltransferases APL_0981 and APJL_1001 are responsible for the transfer of the terminal outer core D-glycero-D-manno-heptose (D,D-Hep) residue although they are not currently included in the CAZY glycosyltransferase 9 family. The WahF heptosyltransferase group signature sequence [S(T/S)(GA)XXH] differs from the heptosyltransferases consensus signature sequence [D(TS)(GA)XXH], because of the substitution of D(261) for S(261), being unique.

The ClpP protease is required for the stress tolerance and biofilm formation in Actinobacillus pleuropneumoniae

In the respiratory tract and lung tissue, a balanced physiological response is essential for Actinobacillus pleuropneumoniae to survive various types of challenges. ClpP, the catalytic core of the Clp proteolytic complex, is involved in various stresses response and regulation of biofilm formation in many pathogenic bacteria. To investigate the role of ClpP in the virulence of A. pleuropneumoniae, the clpP gene was deleted by homologous recombination, resulting in the mutant strain S8ΔclpP. The reduced growth of S8ΔclpP mutant at high temperatures and under several other stress conditions suggests that the ClpP protein is required for the stress tolerance of A. pleuropneumoniae. Interestingly, we observed that the S8ΔclpP mutant exhibited an increased ability to take up iron in vitro compared to the wild-type strain. We also found that the cells without ClpP displayed rough and irregular surfaces and increased cell volume relative to the wild-type strain using scanning electron microscopy (SEM). Confocal laser scanning microscopy (CLSM) revealed that the S8ΔclpP mutant showed decreased biofilm formation compared to the wild-type strain. We examined the transcriptional profiles of the wild type S8 and the S8ΔclpP mutant strains of A. pleuropneumoniae using RNA sequencing. Our analysis revealed that the expression of 16 genes was changed by the deletion of the clpP gene. The data presented in this study illustrate the important role of ClpP protease in the stress response, iron acquisition, cell morphology and biofilm formation related to A. pleuropneumoniae and further suggest a putative role of ClpP protease in virulence regulation.

Induction of protective immune responses against challenge of Actinobacillus pleuropneumoniae by oral administration with Saccharomyces cerevisiae expressing Apx toxins in pigs

Actinobacillus pleuropneumoniae is a causative agent of porcine pleuropneumonia, a highly contagious endemic disease of pigs worldwide, inducing significant economic losses worldwide. Apx toxins, which are correlated with the virulence of A. pleuropneumoniae, were expressed in Saccharomyces cerevisiae and its possible use as an oral vaccine has been confirmed in our previous studies using a murine model. The present study was undertaken to test the hypothesis that oral immunization using S. cerevisiae expressing either ApxI or ApxII could protect pigs against A. pleuropneumoniae as an effective way of inducing both mucosal and systemic immune responses. The surface-displayed ApxIIA#5 expressing S. cerevisiae was selected as an oral vaccine candidate by finding on induction of higher immune responses in mice after oral vaccination. The surface-displayed ApxIIA#5 expressing S. cerevisiae and the ApxIA expressing S. cerevisiae were developed to serve as an oral vaccine in pigs. The vaccinated pigs showed higher specific IgG- and IgA-related antibody activities than the non-treated control and vector control pigs. Additionally, the induced immune responses were found to protect pigs infected with A. pleuropneumoniae according to the analysis of clinical signs and the gross and microscopic pulmonary lesions. These results suggested that the surface-displayed ApxIIA#5 and ApxIA in S. cerevisiae might be a potential oral vaccine to protect pigs against porcine pleuropneumonia. Thus the present study is expected to contribute to the development of a live oral vaccine against porcine pleuropneumonia as an alternative to current conventional vaccines.

Magnetic beads-based enzymatic spectrofluorometric assay for rapid and sensitive detection of antibody against ApxIVA of Actinobacillus pleuropneumoniae

In this paper, a simple, easily-operated and enzyme-amplified fluorescence immunoassay method using magnetic particles for the detection of antibody against Actinobacillus pleuropneumoniae (APP) has been presented. The A protein of APP Repeats-in-Toxin IV (ApxIVA) with high specificity to the APP species was immobilized onto the magnetic bead surfaces. Horseradish peroxidase (HRP), which can catalyze the substrate 4-hydroxyphenylacetic acid (p-HPA), generating fluorescent bi-p, p'-hydroxyphenylacetic acid (DBDA), was selected as an enzymatic-amplified tracer. The ApxIVA antibody was detected for the presence of APP infection by measuring the fluorescence intensity of DBDA. Under optimal conditions, the calibration plot obtained for standard positive serum was approximately linear within the dilution range 1:160-1:5120. The limit of detection (LOD) for the assay was 1:10240, considerably lower than that of ApxIVA-ELISA (1:320) (S/N=3). A series of repeatability measurements of using 1:320-fold diluted standard positive serum gave reproducible results with a relative standard deviation (RSD) of 4.8% (n=11). The ability of the immunosensor to analyze clinical samples was tested on porcine sera. The immunosensor yielded an efficiency of 89.7%, sensitivity of 90.9% and specificity of 89.3% compared with ApxIVA-ELISA.

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.

Functional pentameric formation via coexpression of the Escherichia coli heat-labile enterotoxin B subunit and its fusion protein subunit with a neutralizing epitope of ApxIIA exotoxin improves the mucosal immunogenicity and protection against challenge by Actinobacillus pleuropneumoniae

A coexpression strategy in Saccharomyces cerevisiae using episomal and integrative vectors for the Escherichia coli heat-labile enterotoxin B subunit (LTB) and a fusion protein of an ApxIIA toxin epitope produced by Actinobacillus pleuropneumoniae coupled to LTB, respectively, was adapted for the hetero-oligomerization of LTB and the LTB fusion construct. Enzyme-linked immunosorbent assay (ELISA) with GM1 ganglioside indicated that the LTB fusion construct, along with LTB, was oligomerized to make the functional heteropentameric form, which can bind to receptors on the mucosal epithelium. The antigen-specific antibody titer of mice orally administered antigen was increased when using recombinant yeast coexpressing the pentameric form instead of recombinant yeast expressing either the LTB fusion form or antigen alone. Better protection against challenge infection with A. pleuropneumoniae was also observed for coexpression in recombinant yeast compared with others. The present study clearly indicated that the coexpression strategy enabled the LTB fusion construct to participate in the pentameric formation, resulting in an improved induction of systemic and mucosal immune responses.

An immunosorbent assay based on the recombinant ApxIa, ApxIIa, and ApxIIIa toxins of Actinobacillus pleuropneumoniae and its application to field sera

Actinobacillus pleuropneumoniae is the etiologic agent of porcine pleuropneumonia, a highly contagious pulmonary disease in pigs with major economic losses for pig producers worldwide. Whereas A. pleuropneumoniae isolates are divided into 15 serotypes, the isolates secrete 4 types of exotoxins (ApxI, ApxII, ApxIII, and ApxIV), which are known as major virulence factors. In the current study, the ApxIA, ApxIIA, and ApxIIIA genes were amplified and their recombinant proteins expressed in Escherichia coli M15 cells. The antigenicity of each recombinant protein was demonstrated by Western blot and enzyme-linked immunosorbent assay (ELISA) using sera from pigs vaccinated with a subunit vaccine. When ELISAs using the recombinant antigens were optimized and then applied to sera from 320 randomized pigs in Korea, an observed increase in seroprevalence was found among sows in comparison with weaned piglets and growing pigs, indicating an age-dependent seroprevalence. The results obtained in the study suggest that the developed ELISAs may be useful for A. pleuropneumoniae vaccination strategy as a screening tool for pig herds as well as for detection of specific antibodies to Apx exotoxins.

Comparative profiling of the transcriptional response to iron restriction in six serotypes of Actinobacillus pleuropneumoniae with different virulence potential

Background

Comparative analysis of gene expression among serotypes within a species can provide valuable information on important differences between related genomes. For the pig lung pathogen Actinobacillus pleuropneumoniae, 15 serotypes with a considerable variation in virulence potential and immunogenicity have been identified. This serotypic diversity can only partly be explained by amount of capsule and differences in the RTX toxin genes in their genomes. Iron acquisition in vivo is an important bacterial function and in pathogenic bacteria, iron-limitation is often a signal for the induction of virulence genes. We used a pan-genomic microarray to study the transcriptional response to iron restriction in vitro in six serotypes of A. pleuropneumoniae (1, 2, 3, 5b, 6, and 7), representing at least two levels of virulence.

Results

In total, 45 genes were significantly (p < 0.0001) up-regulated and 67 genes significantly down-regulated in response to iron limitation. Not previously observed in A. pleuropneumoniae was the up-regulation of a putative cirA-like siderophore in all six serotypes. Three genes, recently described in A. pleuropneumoniae as possibly coding for haemoglobin-haptoglobin binding proteins, displayed significant serotype related up-regulation to iron limitation. For all three genes, the expression appeared at its lowest in serotype 3, which is generally considered one of the least virulent serotypes of A. pleuropneumoniae. The three genes share homology with the hmbR haemoglobin receptor of Neisseria meningitidis, a possible virulence factor which contributes to bacterial survival in rats.

Conclusions

By comparative analysis of gene expression among 6 different serotypes of A. pleuropneumoniae we identified a common set of presumably essential core genes, involved in iron regulation. The results support and expand previous observations concerning the identification of new potential iron acquisition systems in A. pleuropneumoniae, showing that this bacterium has evolved several strategies for scavenging the limited iron resources of the host. The combined effect of iron-depletion and serotype proved to be modest, indicating that serotypes of both moderate and high virulence at least in vitro are reacting almost identical to iron restriction. One notable exception, however, is the haemoglobin-haptoglobin binding protein cluster which merits further investigation.

Evaluation of multicomponent recombinant vaccines against Actinobacillus pleuropneumoniae in mice

Background

Porcine contagious pleuropneumonia (PCP) is a highly contagious disease that is caused by Actinobacillus pleuropneumoniae (APP) and characterized by severe fibrinous necrotizing hemorrhagic pleuropneumonia, which is a severe threat to the swine industry. In addition to APP RTX-toxins I (ApxI), APP RTX-toxin II (ApxII), APP RTX-toxin III (ApxIII) and Outer membrane protein (OMP), there may be other useful antigens that can contribute to protection. In the development of an efficacious vaccine against APP, the immunogenicities of multicomponent recombinant subunit vaccines were evaluated.

Methods

Six major virulent factor genes of APP, i.e., apxI, apxII, apxIII, APP RTX-toxins IV (apxIV), omp and type 4 fimbrial structural (apfa) were expressed. BALB/c mice were immunized with recombinant ApxI ( rApxI), recombinant ApxII (rApxII), recombinant ApxIII (rApxIII) and recombinant OMP (rOMP) (Group I); rApxI, rApxII, rApxIII, recombinant ApxIV (rApxIV), recombinant Apfa (rApfa) and rOMP (Group II); APP serotype 1 (APP1) inactivated vaccine (Group III); or phosphate-buffered saline (PBS) (Control group), respectively. After the first immunization, mice were subjected to two booster immunizations at 2-week intervals, followed by challenge with APP1 Shope 4074 and APP2 S1536.

Results

The efficacy of the multicomponent recombinant subunit vaccines was evaluated on the basis of antibody titers, survival rates, lung lesions and indirect immunofluorescence (IIF) detection of APP. The antibody level of Group I was significantly higher than those of the other three groups (P < 0.05). The survival rate of Group I was higher than that of Groups II and III (P < 0.05) and the control (P < 0.01). Compared with the other three groups, the lungs of Group I did not exhibit obvious hemorrhage or necrosis, and only showed weak and scattered fluorescent dots by IIF detection.

Conclusion

The result indicates that the multicomponent recombinant subunit vaccine composed of rApxI, rApxII, rApxIII and rOMP can provide effective cross-protection against homologous and heterologous APP challenge.

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.

Virulence factors of Actinobacillus pleuropneumoniae involved in colonization, persistence and induction of lesions in its porcine host

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia. The virulence factors of this microorganism involved in colonization and the induction of lung lesions have been thoroughly studied and some have been well characterized. A. pleuropneumoniae binds preferentially to cells of the lower respiratory tract in a process involving different adhesins and probably biofilm formation. Apx toxins and lipopolysaccharides exert pathogenic effects on several host cells, resulting in typical lung lesions. Lysis of host cells is essential for the bacterium to obtain nutrients from the environment and A. pleuropneumoniae has developed several uptake mechanisms for these nutrients. In addition to persistence in lung lesions, colonization of the upper respiratory tract – and of the tonsils in particular – may also be important for long-term persistent asymptomatic infection. Information on virulence factors involved in tonsillar and nasal cavity colonization and persistence is scarce, but it can be speculated that similar features as demonstrated for the lung may play a role.

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.

Branched-chain amino acids are required for the survival and virulence of Actinobacillus pleuropneumoniae in swine

In Actinobacillus pleuropneumoniae, which causes porcine pleuropneumonia, ilvI was identified as an in vivo-induced (ivi) gene and encodes the enzyme acetohydroxyacid synthase (AHAS) required for branched-chain amino acid (BCAA) biosynthesis. ilvI and 7 of 32 additional ivi promoters were upregulated in vitro when grown in chemically defined medium (CDM) lacking BCAA. Based on these observations, we hypothesized that BCAA would be found at limiting concentrations in pulmonary secretions and that A. pleuropneumoniae mutants unable to synthesize BCAA would be attenuated in a porcine infection model. Quantitation of free amino acids in porcine pulmonary epithelial lining fluid showed concentrations of BCAA ranging from 8 to 30 micromol/liter, which is 10 to 17% of the concentration in plasma. The expression of both ilvI and lrp, a global regulator that is required for ilvI expression, was strongly upregulated in CDM containing concentrations of BCAA similar to those found in pulmonary secretions. Deletion-disruption mutants of ilvI and lrp were both auxotrophic for BCAA in CDM and attenuated compared to wild-type A. pleuropneumoniae in competitive index experiments in a pig infection model. Wild-type A. pleuropneumoniae grew in CDM+BCAA but not in CDM-BCAA in the presence of sulfonylurea AHAS inhibitors. These results clearly demonstrate that BCAA availability is limited in the lungs and support the hypothesis that A. pleuropneumoniae, and potentially other pulmonary pathogens, uses limitation of BCAA as a cue to regulate the expression of genes required for survival and virulence. These results further suggest a potential role for AHAS inhibitors as antimicrobial agents against pulmonary pathogens.

Effects of growth conditions on biofilm formation by Actinobacillus pleuropneumoniae

Biofilm formation is an important virulence trait of many bacterial pathogens. It has been reported in the literature that only two of the reference strains of the swine pathogen Actinobacillus pleuropneumoniae, representing serotypes 5b and 11, were able to form biofilm in vitro. In this study, we compared biofilm formation by the serotype 1 reference strain S4074 of A. pleuropneumoniae grown in five different culture media. We observed that strain S4074 of A. pleuropneumoniae is able to form biofilms after growth in one of the culture conditions tested brain heart infusion (BHI medium, supplier B). Confocal laser scanning microscopy using a fluorescent probe specific to the poly-N-acetylglucosamine (PGA) polysaccharide further confirmed biofilm formation. In accordance, biofilm formation was susceptible to dispersin B, a PGA hydrolase. Transcriptional profiles of A. pleuropneumoniae S4074 following growth in BHI-B, which allowed a robust biofilm formation, and in BHI-A, in which only a slight biofilm formation was observed, were compared. Genes such as tadC, tadD, genes with homology to autotransporter adhesins as well as genes pgaABC involved in PGA biosynthesis and genes involved in zinc transport were up-regulated after growth in BHI-B. Interestingly, biofilm formation was inhibited by zinc, which was found to be more present in BHI-A (no or slight biofilm) than in BHI-B. We also observed biofilm formation in reference strains representing serotypes 3, 4, 5a, 12 and 14 as well as in 20 of the 37 fresh field isolates tested. Our data indicate that A. pleuropneumoniae has the ability to form biofilms under appropriate growth conditions and transition from a biofilm-positive to a biofilm-negative phenotype was reversible.