Endobronchial inoculation with Apx toxins of Actinobacillus pleuropneumoniae leads to pleuropneumonia in pigs

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.

New Insights into Neutrophil Extracellular Trap (NETs) Formation from Porcine Neutrophils in Response to Bacterial Infections

Actinobacillus pleuropneumoniae (A.pp, Gram negative) and Streptococcus (S.) suis (Gram positive) can cause severe diseases in pigs. During infection, neutrophils infiltrate to counteract these pathogens with phagocytosis and/or neutrophil extracellular traps (NETs). NETs consist of a DNA-backbone spiked with antimicrobial components. The NET formation mechanisms in porcine neutrophils as a response to both of the pathogens are not entirely clear. The aim of this study was to investigate whether A.pp (serotype 2, C3656/0271/11) and S. suis (serotype 2, strain 10) induce NETs by NADPH oxidase- or CD18-dependent mechanisms and to characterize phenotypes of NETs in porcine neutrophils. Therefore, we investigated NET induction in porcine neutrophils in the presence and absence of NET inhibitors and quantified NETs after 3 h. Furthermore, NETosis and phagocytosis were investigated by transmission electron microscopy after 30 min to characterize different phenotypes. A.pp and S. suis induce NETs that are mainly ROS-dependent. A.pp induces NETs that are partially CD18-dependent. Thirty minutes after infection, both of the pathogens induced a vesicular NET formation with only slight differences. Interestingly, some neutrophils showed only NET-marker positive phagolysosomes, but no NET-marker positive vesicles. Other neutrophils showed vesicular NETs and only NET-marker negative phagolysosomes. In conclusion, both of the pathogens induce ROS-dependent NETs. Vesicular NETosis and phagocytosis occur in parallel in porcine neutrophils in response to S. suis serotype 2 and A.pp serotype 2.

Application of the MISTEACHING(S) disease susceptibility framework to Actinobacillus pleuropneumoniae to identify research gaps: an exemplar of a veterinary pathogen

Historically, the MISTEACHING (microbiome, immunity, sex, temperature, environment, age, chance, history, inoculum, nutrition, genetics) framework to describe the outcome of host-pathogen interaction, has been applied to human pathogens. Here, we show, using Actinobacillus pleuropneumoniae as an exemplar, that the MISTEACHING framework can be applied to a strict veterinary pathogen, enabling the identification of major research gaps, the formulation of hypotheses whose study will lead to a greater understanding of pathogenic mechanisms, and/or improved prevention/therapeutic measures. We also suggest that the MISTEACHING framework should be extended with the inclusion of a 'strain' category, to become MISTEACHINGS. We conclude that the MISTEACHINGS framework can be applied to veterinary pathogens, whether they be bacteria, fungi, viruses, or parasites, and hope to stimulate others to use it to identify research gaps and to formulate hypotheses worthy of study with their own pathogens.

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.

Infectious diseases dynamics in growing/finishing pigs in Southern Brazil (2005-2016)

ABSTRACT: This study aimed to determine the frequency and distribution of infectious diseases diagnosed through necropsy examination and histopathological analysis in growing/finishing pigs along 12 years (2005-2016) in Southern Brazil. We evaluated 1906 anatomopathological exams of pigs at growing/finishing phases, of which the infectious diseases corresponded to 75.6% of the cases (1,441/1,906). Porcine circovirus type 2 (PCV2) infections were the most frequent, accounting for 51.3% of the cases (739/1,441) with a higher frequency from 2005 to 2007, characterizing an epidemic distribution, with a gradual decline after 2008. Infectious diseases affecting the respiratory system were the second major cause with 30.1% of the cases. Among these, necrotizing bronchiolitis caused by swine Influenza (15.1%, 218/1,441) and bacterial pneumonia (15%, 216/1,441) were the main conditions. Influenza was mostly diagnosed from 2010 to 2013, accounting for 43.1% (167/387) of the cases. After this period, both respiratory infectious diseases were endemic. Digestive system infectious diseases accounted for 10.5% of the diagnoses (151/1,441), with the following main conditions: Salmonella spp. enterocolitis (43.7%, 66/151), Lawsonia spp. proliferative enteropathy (41.7%, 63/151), and Brachyspira spp. colitis (14.6%, 22/151). The latter had a higher incidence from 2012 to 2014 with all cases detected in this period. Polyserositis and bacterial meningitis represented, respectively, 5.8% (84/1,441) and 2.3% (33/1,441) of the cases diagnosed, with a constant endemic character.

GtxA is a virulence factor that promotes a Th2-like response during Gallibacterium anatis infection in laying hens

GtxA, a leukotoxic RTX-toxin, has been proposed a main virulence factor of Gallibacterium anatis. To evaluate the impact of GtxA during infection, we experimentally infected laying hens with a G. anatis wild-type (WT) strain and its isogenic gtxA deletion mutant (ΔgtxA), respectively, and monitored the birds during a 6 day period. Birds inoculated with ΔgtxA had significantly reduced gross lesions and microscopic changes compared to the birds inoculated with the WT strain. To assess the host response further, we quantified the expression of pro-inflammatory cytokines and apoptosis genes by RT-qPCR. In the ovarian tissue, the expression levels of IL-4 and TNF-α were significantly lower in the ΔgtxA group compared to the WT group, while IL-6 and IL-10 levels appeared similar in the two groups. In the spleen tissue of ΔgtxA infected chickens, IL-4 expression was also lower compared to the WT infected chickens. The results indicated that GtxA plays a key role in an acute cytokine-mediated Th2-like response against G. anatis infection in the ovary tissue. The pro-inflammatory response in the ovary tissue of birds inoculated with ΔgtxA mutant was thus significantly lower than the wild-type response. This was, at least partly, supported by the apoptosis gene expression levels, which were significantly higher in the ΔgtxA mutant compared to the wild-type infected chickens. In conclusion, GtxA clearly plays an important role in the pathogenesis of G. anatis infections in laying hens. Further investigations into the specific factors regulating the host response is however needed to provide a more complete understanding of the bacteria-host interaction.

Isolation and characterization of atypical Actinobacillus pleuropneumoniae serovar 15 lacking the apxIICA genes in Japan

Six atypical Actinobacillus pleuropneumoniae serovar 15 strains were isolated from pneumonic lesions of naturally infected dead pigs from the same farm in Japan. Genetic analyses of apx genes revealed that the atypical isolates contained the toxin-associated genes apxIBD, apxIIICA, apxIIIBD, and apxIVA, but not apxIICA. Coinciding with the result of the atypical gene profile, analyses of toxin protein production revealed that these atypical isolates expressed only ApxIII but not ApxII. A mouse pathogenicity test showed that the atypical isolate tested seemed to be less virulent than the typical isolates. This is the first report describing the emergence of atypical A. pleuropneumoniae serovar 15, which does not produce ApxII due to the absence of apxIICA genes, in Japan.

Detection of Actinobacillus Pleuropneumoniae ApxIV Toxin Antibody in Serum and Oral Fluid Specimens from Pigs Inoculated Under Experimental Conditions

Introduction

The prevention and control of Actinobacillus pleuropneumoniae in commercial production settings is based on serological monitoring. Enzyme-linked immunosorbent assays (ELISAs) have been developed to detect specific antibodies against a variety of A. pleuropneumoniae antigens, including long-chain lipopolysaccharides (LPS) and the ApxIV toxin, a repeats-in-toxin (RTX) exotoxin unique to A. pleuropneumoniae and produced by all serovars. The objective of this study was to describe ApxIV antibody responses in serum and oral fluid of pigs.

Material and Methods

Four groups of pigs (six pigs per group) were inoculated with A. pleuropneumoniae serovars 1, 5, 7, or 12. Weekly serum samples and daily oral fluid samples were collected from individual pigs for 56 days post inoculation (DPI) and tested by LPS and ApxIV ELISAs. The ApxIV ELISA was run in three formats to detect immunlgobulins M, G, and A (IgM, IgG and IgA) while the LPS ELISA detected only IgG.

Results

All pigs inoculated with A. pleuropneumoniae serovars 1 and 7 were LPS ELISA serum antibody positive from DPI 14 to 56. A transient and weak LPS ELISA antibody response was observed in pigs inoculated with serovar 5 and a single antibody positive pig was observed in serovar 12 at ≥35 DPI. Notably, ApxIV serum and oral fluid antibody responses in pig inoculated with serovars 1 and 7 reflected the patterns observed for LPS antibody, albeit with a 14 to 21 day delay.

Conclusion

This work suggests that ELISAs based on ApxIV antibody detection in oral fluid samples could be effective in population monitoring for A. pleuropneumoniae.

Genetic and antigenic characteristics of ApxIIA and ApxIIIA from Actinobacillus pleuropneumoniae serovars 2, 3, 4, 6, 8 and 15

Apx toxins produced by Actinobacillus pleuropneumoniae are essential components of new generation vaccines. In this study, apxIIA and apxIIIA genes of serovars 2, 3, 4, 6, 8 and 15 were cloned and sequenced. Amino acid sequences of ApxIIA proteins of serovars 2, 3, 4, 6, 8 and 15 were almost identical to those of serovars 1, 5, 7, 9 and 11-13. Immunoblot analysis showed that rApxIIA from serovars 2 and 15 reacts strongly with sera from animals infected with various serovars. Sequence analysis revealed that ApxIIIA proteins has two variants, one in strains of serovar 2 and the other in strains of serovars 3, 4, 6, 8 and 15. A mouse cross-protection study showed that mice actively immunized with rApxIIIA/2 or rApxIIIA/15 are protected against challenge with A. pleuropneumoniae strains of serovars 3, 4, 6, 8, 15, and 2 expressing ApxIII/15 and ApxIII/2, respectively. Similarly, mice passively immunized with rabbit anti-rApxIIIA/2 or anti-rApxIIIA/15 sera were found to be protected against challenge with strains of serovars 2 and 15. Our study revealed antigenic and sequence similarities within ApxIIA and ApxIIIA proteins, which may help in the development of effective vaccines against disease caused by A. pleuropneumoniae.

Biology and Diseases of Swine

Swine are used in biomedical research as models for biomedical research and for teaching. This chapter covers normative biology and behavior along with common and emerging swine diseases. Xenotransplantation is discussed along with similarities and differences of swine immunology.

Elucidating the role of ApxI in hemolysis and cellular damage by using a novel apxIA mutant of Actinobacillus pleuropneumoniae serotype 10

Exotoxins produced by Actinobacillus (A.) pleuropneumoniae (Apx) play major roles in the pathogenesis of pleuropneumonia in swine. This study investigated the role of ApxI in hemolysis and cellular damage using a novel apxIA mutant, ApxIA336, which was developed from the parental strain A. pleuropneumoniae serotype 10 that produces only ApxI in vitro. The genotype of ApxIA336 was confirmed by PCR, Southern blotting, and gene sequencing. Exotoxin preparation derived from ApxIA336 was analyzed for its bioactivity towards porcine erythrocytes and alveolar macrophages. Analysis results indicated that ApxIA336 contained a kanamycin- resistant cassette inserted immediately after 1005 bp of the apxIA gene. Phenotype analysis of ApxIA336 revealed no difference in the growth rate as compared to the parental strain. Meanwhile, ApxI production was abolished in the bacterial culture supernatant, i.e. exotoxin preparation. The inability of ApxIA336 to produce ApxI corresponded to the loss of hemolytic and cytotoxic bioactivity in exotoxin preparation, as demonstrated by hemolysis, lactate dehydrogenase release, mitochondrial activity, and apoptosis assays. Additionally, the virulence of ApxIA336 appeared to be attenuated by 15-fold in BALB/c mice. Collectively, ApxI, but not other components in the exotoxin preparation of A. pleuropneumoniae serotype 10, was responsible for the hemolytic and cytotoxic effects on porcine erythrocytes and alveolar macrophages.

Polymicrobial respiratory disease in pigs

Respiratory disease in pigs is common in modern pork production worldwide and is often referred to as porcine respiratory disease complex (PRDC). PRDC is polymicrobial in nature, and results from infection with various combinations of primary and secondary respiratory pathogens. As a true multifactorial disease, environmental conditions, population size, management strategies and pig-specific factors such as age and genetics also play critical roles in the outcome of PRDC. While non-infectious factors are important in the initiation and outcome of cases of PRDC, the focus of this review is on infectious factors only. There are a variety of viral and bacterial pathogens commonly associated with PRDC including porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), porcine circovirus type 2 (PCV2), Mycoplasma hyopneumoniae (MHYO) and Pasteurella multocida (PMULT). The pathogenesis of viral respiratory disease is typically associated with destruction of the mucocilliary apparatus and with interference and decrease of the function of pulmonary alveolar and intravascular macrophages. Bacterial pathogens often contribute to PRDC by activation of inflammation via enhanced cytokine responses. With recent advancements in pathogen detection methods, the importance of polymicrobial disease has become more evident, and identification of interactions of pathogens and their mechanisms of disease potentiation has become a topic of great interest. For example, combined infection of pigs with typically low pathogenic organisms like PCV2 and MHYO results in severe respiratory disease. Although the body of knowledge has advanced substantially in the last 15 years, much more needs to be learned about the pathogenesis and best practices for control of swine respiratory disease outbreaks caused by concurrent infection of two or more pathogens. This review discusses the latest findings on polymicrobial respiratory disease in pigs.

Actinobacillus pleuropneumoniae is impaired by the garlic volatile allyl methyl sulfide (AMS) in vitro and in-feed garlic alleviates pleuropneumonia in a pig model

Decomposition products of ingested garlic are to a certain extent excreted via the lungs. If the supposed health-supporting capacities associated with garlic extend to these exhaled sulfurous compounds, they could have an effect on the course of pneumonia. In this study, the garlic-derived volatile allyl methyl sulfide (AMS) as a lead compound of volatile garlic metabolites was shown to exhibit an antibacterial effect against the pig pathogen Actinobacillus pleuropneumoniae serotype 9. AMS caused a delay in the appearance of the optical density-monitored growth of A. pleuropneumoniae in medium when compared to unaffected growth curves, yet without lowering the stationary phase yield at the concentration range tested. At 1.1mM, AMS impaired the in vitro growth rate of A. pleuropneumoniae serotype 9 by 8% compared to unimpeded growth. In an animal trial, a garlic-fed group of 15 pigs that received a diet with 5% garlic feed component and a control group of 15 pigs that received a diet without garlic were infected with A. pleuropneumoniae serotype 2 via an aerosol and subsequently followed for 4 days. At the day of the challenge, blood AMS in the garlic-fed group amounted to 0.32 ± 0.13 μM. A beneficial, alleviating effect of garlic on the course and severity of an A. pleuropneumoniae infection in pigs was indicated by the reduced occurrence of characteristic pleuropneumonia lesions (27% of the lungs affected in the garlic-fed group vs. 47% in the control group) and a near to significant (p=0.06) lower relative lung weight post mortem in the garlic-fed group.

The role of RTX toxins in host specificity of animal pathogenic Pasteurellaceae

RTX toxins are bacterial pore-forming toxins that are particularly abundant among pathogenic species of Pasteurellaceae, in which they play a major role in virulence. RTX toxins of several primary pathogens of the family of Pasteurellaceae are directly involved in causing necrotic lesions in the target organs. Many RTX toxins are known as haemolysins because they lyse erythrocytes in vitro, an effect that is non-specific, but which serves as a useful marker in bacteriological identification and as an easily measurable signal in vitro in experimental studies. More recent studies have shown that the specific targets of most RTX toxins are leukocytes, with RTX toxins binding to the corresponding β-subunit (CD18) of β2 integrins and then exerting cytotoxic activity. After uptake by the target cell, at sub-lytic concentrations, some RTX toxins are transported to mitochondria and induce apoptosis. For several RTX toxins the binding to CD18 has been shown to be host specific and this seems to be the basis for the host range specificity of these RTX toxins. Observations on two very closely related species of the Pasteurellaceae family, Actinobacillus suis, a porcine pathogen particularly affecting suckling pigs, and Actinobacillus equuli subsp. haemolytica, which causes pyosepticaemia in new-born foals (sleepy foal disease), have revealed that they express different RTX toxins, named ApxI/II and Aqx, respectively. These RTX toxins are specifically cytotoxic for porcine and equine leukocytes, respectively. Furthermore, the ApxI and Aqx toxins of these species, when expressed in an isogenetic background in Escherichia coli, are specifically cytotoxic for leukocytes of their respective hosts. These data indicate the determinative role of RTX toxins in host specificity of pathogenic species of Pasteurellaceae.

Porcine CD18 mediates Actinobacillus pleuropneumoniae ApxIII species-specific toxicity

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, produces Apx toxins that are recognized as major virulence factors. Recently, we showed that ApxIIIA-cytotoxic activity specifically targets Sus scrofa leukocytes. Since both LtxA from Aggregatibacter actinomycetemcomitans (aggressive periodontitis in humans) and LktA from Mannheimia haemolytica (pneumonia in ruminants) share this characteristic, respectively towards human and ruminant leukocytes, and because both use the CD18 subunit to interact with their respective LFA-1, we hypothesized that ApxIIIA was likely to bind porcine CD18 to exercise its deleterious effects on pig leukocytes. A β₂−integrin-deficient ApxIIIA-resistant human erythroleukemic cell line was transfected either with homologous or heterologous CD11a/CD18 heterodimers using a set of plasmids coding for human (ApxIIIA-resistant), bovine (-resistant) and porcine (-susceptible) CD11a and CD18 subunits. Cell preparations that switched from ApxIIIA-resistance to -susceptibility were then sought to identify the LFA-1 subunit involved. The results showed that the ApxIIIA-resistant recipient cell line was rendered susceptible only if the CD18 partner within the LFA-1 heterodimer was that of the pig. It is concluded that porcine CD18 is necessary to mediate A. pleuropneumoniae ApxIIIA toxin-induced leukolysis.

In vivo induced RTX toxin ApxIVA is essential for the full virulence of Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae is a Gram-negative pathogen. It is the aetiological agent of porcine contagious pleuropneumonia (PCP), a severe and highly contagious and severe respiratory disease of swine. Four sets of RTX (repeats in toxin) exotoxins have been described in A. pleuropnuemoniae and three of them have been characterized as important virulence determinants. The aim of this study was to determine the pathogenicity of the invivo induced RTX toxin ApxIVA during infection of piglets with A. pleuropnuemoniae. An A. pleuropnuemoniae apxIVA mutant was obtained based on an A. pleuropnuemoniae apxIIC-deleted mutant strain. An experimental infection assay was performed to evaluate the virulence of ApxIVA in piglets. Clinical signs, lung lesion scores, blood biochemical parameters and histopathologic changes in the piglets were recorded. The results indicated that the pathogenicity of A. pleuropnuemoniae was greater when ApxIVA was present, suggesting that ApxIVA is essential for expression of the full virulence of A. pleuropnuemoniae.

Probing of Actinobacillus pleuropneumoniae ApxIIIA toxin-dependent cytotoxicity towards mammalian peripheral blood mononucleated cells

Background

Actinobacillus pleuropneumoniae, the causative bacterial agent of porcine pleuropneumonia, produces Apx toxins which belong to RTX toxin family and are recognized as the major virulence factors. So far, their target receptor(s) has not been identified and the disease cytopathogenesis remains poorly understood. Production of an active Apx toxin and characterization of its toxic activity constitute the premises necessary to the description of its interaction with a potential receptor. From this point of view, we produced an active recombinant ApxIIIA toxin in order to characterize its toxicity on peripheral blood mononucleated cells (PBMCs) isolated from several species.

Findings

Toxin preparation exercises a strong cytotoxic action on porcine PBMCs which is directly related to recombinant ApxIIIA since preincubation with polymyxin B does not modify the cytotoxicity rate while preincubation with a monospecific polyclonal antiserum directed against ApxIIIA does. The cell death process triggered by ApxIIIA is extremely fast, the maximum rate of toxicity being already reached after 20 minutes of incubation. Moreover, ApxIIIA cytotoxicity is species-specific because llama, human, dog, rat and mouse PBMCs are resistant. Interestingly, bovine and caprine PBMCs are slightly sensitive to ApxIIIA toxin too. Finally, ApxIIIA cytotoxicity is cell type-specific as porcine epithelial cells are resistant.

Conclusion

We have produced an active recombinant ApxIIIA toxin and characterized its specific cytotoxicity on porcine PBMCs which will allow us to get new insights on porcine pleuropneumonia pathogenesis in the future.

Genome biology of Actinobacillus pleuropneumoniae JL03, an isolate of serotype 3 prevalent in China

Actinobacillus pleuropneumoniae is the etiologic agent of porcine contagious pleuropneumonia, a cause of considerable world wide economic losses in the swine industry. We sequenced the complete genome of A. pleuropneumoniae, JL03, an isolate of serotype 3 prevalent in China. Its genome is a single chromosome of 2,242,062 base pairs containing 2,097 predicted protein-coding sequences, six ribosomal rRNA operons, and 63 tRNA genes. Preliminary analysis of the genomic sequence and the functions of the encoded proteins not only confirmed the present physiological and pathological knowledge but also offered new insights into the metabolic and virulence characteristics of this important pathogen. We identified a full spectrum of genes related to its characteristic chemoheterotrophic catabolism of fermentation and respiration with an incomplete TCA system for anabolism. In addition to confirming the lack of ApxI toxin, identification of a nonsense mutation in apxIVA and a 5'-proximal truncation of the flp operon deleting both its promoter and the flp1flp2tadV genes have provided convincing scenarios for the low virulence property of JL03. Comparative genomic analysis using the available sequences of other serotypes, probable strain (serotype)-specific genomic islands related to capsular polysaccharides and lipopolysaccharide O-antigen biosyntheses were identified in JL03, which provides a foundation for future research into the mechanisms of serotypic diversity of A. pleuropneumoniae.

Transcriptional profiling of Actinobacillus pleuropneumoniae under iron-restricted conditions

Background

To better understand effects of iron restriction on Actinobacillus pleuropneumoniae and to identify new potential vaccine targets, we conducted transcript profiling studies using a DNA microarray containing all 2025 ORFs of the genome of A. pleuropneumoniae serotype 5b strain L20. This is the first study involving the use of microarray technology to monitor the transcriptome of A. pleuropneumoniae grown under iron restriction.

Results

Upon comparing growth of this pathogen in iron-sufficient versus iron-depleted medium, 210 genes were identified as being differentially expressed. Some genes (92) were identified as being up-regulated; many have confirmed or putative roles in iron acquisition, such as the genes coding for two TonB energy-transducing proteins and the hemoglobin receptor HgbA. Transcript profiling also led to identification of some new iron acquisition systems of A. pleuropneumoniae. Genes coding for a possible Yfe system (yfeABCD), implicated in the acquisition of chelated iron, were detected, as well as genes coding for a putative enterobactin-type siderophore receptor system. ORFs for homologs of the HmbR system of Neisseria meningitidis involved in iron acquisition from hemoglobin were significantly up-regulated. Down-regulated genes included many that encode proteins containing Fe-S clusters or that use heme as a cofactor. Supplementation of the culture medium with exogenous iron re-established the expression level of these genes.

Conclusion

We have used transcriptional profiling to generate a list of genes showing differential expression during iron restriction. This strategy enabled us to gain a better understanding of the metabolic changes occurring in response to this stress. Many new potential iron acquisition systems were identified, and further studies will have to be conducted to establish their role during iron restriction.

Renaturation and purification of ApxII toxin of Actinobacillus pleuropneumoniae

ApxII toxin is the only Apx toxin that is produced by Actinobacillus pleuropneumoniae serotype 7. In order to determine whether the recombinant ApxII that derived from Escherichia coli (E. coli) expression is faithful to the natural ApxII so that can be used as additional component in vaccine preparation, the structure gene apxIIA of ApxII toxin was expressed in E. coli with prokaryotic expression vector pGEX-6p-1 (formed pGEX-6p-A). pGZRS-C which is A. pleuropneumoniae-E. coli shuttle vector pGZRS-38 expressing the post-transcriptional activation gene apxII C was co-expressed with pGEX-6p-A. The expression product of rApxII A formed inclusion. The inclusion protein was oxidized, refolded and restored hemolytic activity after denaturation, renaturation and purification. The result indicated that E. coli expressed recombinant ApxII toxin has good fidelity, which makes it possible to produce this valuable antigen for vaccine preparation or diagnosis.

Generation of monoclonal antibodies and epitope mapping of ApxIVA of Actinobacillus pleuropneumoniae

To study functions of ApxIV, a species-specific and in vivo inducible RTX toxin identified in Actinobacillus pleuropneumoniae recently, and to develop a diagnostic trial distinguishing the pigs infected naturally and vaccinated with inactivated and/or subunit vaccines, we attempted to prepare monoclonal antibodies against ApxIV. BALB/c mice were immunized with ApxIVAN and ApxIVAC which are N- and C-terminal halvies (814 and 997 amino acids, respectively) of ApxIVA produced in E. coli BL21 (DE3), respectively. Eight monoclonal antibodies were selected, four (designated as 1A8, 1G5, 3E7 and 4H9) against ApxIVAN and another four (named as 1B12, 2E5, 4D8 and 4G2) against ApxIVAC. Western blot and ELISA additivity assays suggested that all monoclonal antibodies except 1A8 are specific to the corresponding immunogen, 1A8 reacts with both immunogens which have a overlapping region of 156 residues. ELISA additivity tests revealed that at least five epitopes in ApxIV are defined by eight monoclonal antibodies, two between 1 and 866 amino acids, one between 867 and 1022 amino acids and two between 1023 and 1863 amino acids. In conclusion, we have succeeded in producing eight monoclonal antibodies, which react with five different epitopes of ApxIV.

Revised definition of Actinobacillus sensu stricto isolated from animals. A review with special emphasis on diagnosis

The taxonomy of the members of the genus Actinobacillus associated with animals has been reviewed with focus on classification and identification including molecular based characterization, typing and identification. Out of the 22 species or species like taxa reported as Actinobacillus, 19 are associated with animals. When classified on the basis of 16S rRNA sequence based phylogenetic analysis, DNA-DNA hybridizations and phenotypic analysis, Actinobacillus sensu stricto is restricted to include A. lignieresii, A. pleuropneumoniae, A. equuli subsp. equuli, A. equuli subsp. haemolyticus (taxon 11 of Bisgaard), A. hominis, A. suis, A. ureae, A. arthritidis (taxon 9 of Bisgaard), Actinobacillus genomospecies 1 and 2 and the taxa 8 and 26 of Bisgaard. The remaining 11 species of Actinobacillus are unrelated to A. sensu stricto and should consequently be grouped with other genera or be renamed as new genera depending on new data. Identification of members of Actinobacillus at species level is possible through phenotypic characterization combined with information on host of isolation. PCR tests are available for specific detection of A. pleuropneumoniae. Only A. pleuropneumoniae is presently considered as a primary pathogen. Based on different types of RTX genes it is possible to PCR type A. pleuropneumoniae to serotype level. PCR might also be used for the specific detection of A. equuli subsp. haemolyticus. Epidemiological investigations and surveillance have so far included serotyping, multilocus enzyme electrophoresis (MLEE), ribotyping and restriction fragment length profiling.

Both ApxI and ApxII of Actinobacillus pleuropneumoniae serotype 1 are necessary for full virulence

Most serotypes of A. pleuropneumoniae produce more than one toxin in vivo. To determine the value of the production of more than one toxin in the development of disease, we tested the pathogenicity of isogenic strains of A. pleuropneumoniae serotype 1 that are mutated in the toxin genes apxIA and/or apxIIA or in the transport genes apxIBD. Bacteria mutated in both apxIA and apxIIA, or in apxIBD, were unable to induce pathological lesions, thereby confirming the conclusion that ApxI and ApxII are essential for the pathogenesis of pleuropneumonia. Infection with isogenic strains lacking either ApxI or ApxII did not consistently lead to pleuropneumonia unlike the parent strain S4074. ApxII seemed at least as important as ApxI for the development of clinical and pathological symptoms. Only one of the four pigs inoculated with a mutant strain unable to produce ApxII developed mild pneumonia whereas two out of the three pigs inoculated with a mutant strain unable to produce ApxI developed more severe lesions. The results indicate that both ApxI and ApxII of A. pleuropneumoniae serotype 1 are necessary for full virulence.

Expression of Actinobacillus pleuropneumonia gene coding for Apx I protein in Escherichia coli

This study presents cloning and expression of Actinobacillus pleuropneumoniae Apx I toxin in Escherichia coli expression system to produce fusion protein for the subsequent immunological studies. The gene coding Apx I toxin was amplified from the A. pleuropneumoniae serotype 10 DNA using polymerase chain reaction and cloned to vector under the control of strong, inducible T7 promoter. The presence of insert was confirmed by PCR screening and sequencing after the propagation of recombinant DNA in E. coli cells. The gene coding A. pleuropneumoniae Apx I toxin was extended with a segment to encode a polyhistidine tag linked to its C-terminal sequence allowing a one-step affinity purification of the complex with Ni-NTA resin. Expression of the Apx I coding sequence in E. coli resulted in the formation of insoluble inclusion bodies purified according to a standard purification protocol. The ease of this expression system, the powerful single-step purification and low costs make it possible to produce Apx I in large amounts to further study the role of Apx I in physiological processes.

Design and analysis of an Actinobacillus pleuropneumoniae transmission experiment

This paper describes a methodology to quantify the transmission of Actinobacillus (A.) pleuropneumoniae from subclinically infected carrier pigs to susceptible contact pigs, and to test the effect of possible interventions on the transmission. The methodology includes the design of a transmission experiment, and a method with which A. pleuropneumoniae transmission can be quantified and with which the effect of an intervention on the transmission can be tested. The experimental design consists of two parts. First, subclinically infected carrier pigs are created by contact exposure of specific-pathogen-free pigs to endobronchially inoculated pigs. Second, transmission is observed from the group of carrier pigs to a second group of susceptible contact pigs after replacing the inoculated pigs by new contact pigs. The presented analytical method is a generalised linear model (GLM) with which the effect of an intervention on the susceptibility and infectivity can be tested separately, if the transmission is observed in heterogeneous populations. The concept of the experimental transmission model is illustrated by describing an A. pleuropneumoniae transmission experiment in which the effect of vaccination on the susceptibility is quantified. Although it could not be demonstrated that vaccination has an effect on the susceptibility of pigs, it was demonstrated that nasal excretion of A. pleuropneumoniae is related to the infectivity of pigs.

Adherence of Actinobacillus pleuropneumoniae to primary cultures of porcine lung epithelial cells

To study adherence of Actinobacillus pleuropneumoniae to porcine lower respiratory epithelium, a cell culture model was developed using primary cultures of porcine lung epithelial cells (LEC). Adherence assays were performed and results were compared with data obtained with swine kidney cells (SK6). A. pleuropneumoniae efficiently adhered to LEC with up to 62 bacteria per cell after 2h of incubation. Reference strain of serotype 3 (R3) adhered better to LEC than reference strains of serotypes 1 (R1), 7 (R7) and 8 (R8). Overall the adherence to LEC was more rapid and up to 30-fold more efficient than adherence to SK6 cells. In search for the mechanism involved in the adherence event, we tested the effect of LPS which has previously been demonstrated to cause adherence of the pathogen to upper respiratory epithelium. Adherence assays with LPS transposon mutants demonstrated unaltered (mutant with modification in core/lipid A moiety) or even three-fold more adherence (mutants lacking O antigen) compared to the parent micro-organisms. Purified LPS of strains R1, R3, R7 and R8 did not inhibit adherence of R8 to LEC either, suggesting that LPS and particularly the O-antigen are not essential for adherence of A. pleuropneumoniae to LEC. The efficient, LPS-independent adherence of A. pleuropneumoniae to LEC cells indicates that A. pleuropneumoniae may carry different, cell type-specific adhesins and that primary cultures of lower respiratory epithelium are valuable infection models in studying A. pleuropneumoniae pathogenesis.

The N-terminal domain of RTX toxin ApxI of Actinobacillus pleuropneumoniae elicits protective immunity in mice

We expressed three Actinobacillus pleuropneumoniae ApxI deletion derivatives to map the domain that could induce protective immunity. Antiserum to ApxI N-terminal covered by residues 40 to 380 was found to neutralize ApxI hemolytic activity but not ApxIII cytotoxicity. When used as a subunit vaccine in mice, this recombinant N-terminal fragment elicited protection against lethal infection with heterologous A. pleuropneumoniae serovars.

RTX toxins in Pasteurellaceae

RTX toxins (repeats in the structural toxin) are pore-forming protein toxins produced by a broad range of pathogenic Gram-negative bacteria. In vitro, RTX toxins mostly exhibit a cytotoxic and often also a hemolytic activity. They are particularly widespread in species of the family Pasteurellaceae which cause infectious diseases, most frequently in animals but also in humans. Most RTX toxins are proteins with a molecular mass of 100-200 kDa and are post-translationally activated by acylation via a specific activator protein. The repeated structure of RTX toxins, which gave them their name, is composed of iterative glycine-rich nonapeptides binding Ca2+ on the C-terminal half of the protein. Genetic analysis of RTX toxins of various species of Pasteurellaceae and of a few other Gram-negative bacteria gave evidence of horizontal transfer of genes encoding RTX toxins and led to speculations that RTX toxins might have originated from Pasteurellaceae. The toxic activities of RTX toxins in host cells may lead to necrosis and apoptosis and the underlying detailed mechanisms are currently under investigation. The impact of RTX toxins in pathogenicity and the immune responses of the host were described for several species of Pasteurellaceae. Neutralizing antibodies were shown to significantly reduce the cytotoxic activity of RTX toxins. They constitute a valuable strategy in the development of immuno-prophylactics against several animal diseases caused by pathogenic species of Pasteurellaceae. Although many RTX toxins possess cytotoxic and hemolytic activities toward a broad range of cells and erythrocytes, respectively, a few RTX toxins were shown to have cytotoxic activity only against cells of specific hosts and/or show cell-type specificity. Further evidence exists that RTX toxins play a potential role in host specificity of certain pathogens.

Porcine surfactant protein D is N-glycosylated in its carbohydrate recognition domain and is assembled into differently charged oligomers

Surfactant protein D (SP-D) belongs to a subgroup of mammalian collagenous Ca(2+)-dependent lectins known as the collectins. It is thought to play a significant role in the innate immune response against microorganisms within the lungs and at other mucosal surfaces. This report documents the isolation and characterization of SP-D purified from porcine lung lavage using mannan affinity chromatography and gel filtration. Ultrastructural analysis shows both dodecameric and higher order oligomeric complexes of SP-D. The molecular mass of monomeric porcine SP-D (50 kD) is larger than that of SP-D from humans (43 kD). The difference in mass is due to the presence of an Asparagine-linked glycosylation in the carbohydrate recognition domain of porcine SP-D, which is absent in SP-D of other species investigated so far. Analysis of this carbohydrate moiety indicates that it is a highly heterogeneous, complex type oligosaccharide which is sialylated. The heterogeneity of oligosaccharide sialylation results in the existence of many differently charged porcine SP-D isoforms. The removal of the carbohydrate moiety reduces the inhibitory effect of porcine SP-D on influenza A virus haemagglutination. Therefore, the carbohydrate moiety may influence interactions with pathogens.

Actinobacillus pleuropneumoniae: pathobiology and pathogenesis of infection

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

Detection and localization of ApxI, -II and -III genes of Actinobacillus pleuropneumoniae in natural porcine pleuropneumonia in natural porcine pleuropneumonia by in situ hybridization

In situ hybridization techniques that employed a nonradioactive digoxigenin-labeled probe were used to detect and localize ApxI, II and III genes in tissue sections of pneumonic lung naturally infected with Actinobacillus pleuropneumoniae. In pigs infected with either serotype 2 or 6, a hybridization signal for apxIICA, apxIIICA, apxIBD, and apxIIIBD was detected, and in pigs infected with serotype 5, a hybridization signal for apxICA, apxIICA, and apxIBD was detected in the pneumonic lesions. A hybridization signal for apxIICA and apxIBD was detected in pigs infected with serotype 7. A strong hybridization signal for apx genes was seen in streaming degenerate alveolar leukocytes bordering zones of coagulative necrosis. Simultaneous detection of hybridization signals for the apxCA and apxBD genes provided scientific evidence that the expression of the apx genes could be potential indicators of the production of corresponding Apx toxins. This study demonstrates the expression of ApxI, II, and III genes in pneumonic lesions caused by A. pleuropneumoniae.

Intramuscular immunization with genetically inactivated (ghosts) Actinobacillus pleuropneumoniae serotype 9 protects pigs against homologous aerosol challenge and prevents carrier state

Bacterial ghosts are empty cell envelopes achieved by the expression of a cloned bacteriophage lysis gene and, unlike classical bacterins, suffer no denaturing steps during their production. These properties may lead to a superior presentation of surface antigens to the immune system. Currently available porcine Actinobacillus pleuropneumoniae vaccines afford only minimal protection by decreasing mortality but not morbidity. Pigs which survive infection can still be carriers of the pathogen, so a herd once infected remains infected. Carrier pigs harbour A. pleuropneumoniae in their nasal cavities, in their tonsils, or within lung lesions. A dose-defined nose-only aerosol infection model for pigs was used to study the immunogenic and protective potential of systemic immunization with ghosts made from A. pleuropneumoniae serotype 9 reference strain CVI 13261 against an homologous aerogenous challenge. Pigs were vaccinated twice intramuscularly with a dose of 5x10(9) CFU ghosts (GVPs) or formalin-inactivated A. pleuropneumoniae bacterins (BVPs). After 2 weeks vaccinated pigs and non-vaccinated placebo controls (PCs) were challenged with a dose of 10(9) CFU by aerosol. The protective efficacy of immunization was evaluated by clinical, bacteriological, serological and post-mortem examinations. Bronchoalveolar lavage in pigs was performed during the experiment to obtain lavage samples (BALF) for assessment of local antibodies. Isotype-specific antibody responses in serum and BALF were determined by ELISAs based on whole-cell antigen. Immunization with ghosts did not cause clinical side-effects. After aerosol challenge PCs developed fever and pleuropneumonia. GVPs or BVPs were found to be fully protected against clinical disease or lung lesions in both vaccination groups, whereas colonization of the respiratory tract with A. pleuropneumoniae was only prevented in GVPs. Specific immunoglobins against A. pleuropneumoniae were not detectable in BALF after immunization. A significant systemic increase of IgM, IgA, IgG(Fc'), or IgG(H+L) antibodies reactive with A. pleuropneumoniae was measured in GVPs and BVPs when compared to the non-exposed controls. BVPs reached higher titers of IgG(Fc') and IgG(H+L) than GVPs. However, prevention of carrier state in GVPs coincided with a significant increase of serum IgA when compared to BVPs. These results suggest that immunization with ghosts, that bias antibody populations specific to non-denaturated surface antigens, may be more efficacious in protecting pigs against colonization and infection than bacterins.

Apx toxins in Pasteurellaceae species from animals

Pasteurellaceae species particularly of porcine origin which are closely related to Actinobacillus pleuropneumoniae were analyzed for the presence of analogues to the major A. pleuropneumoniae RTX toxin genes, apxICABD, apxIICA and apxIIICABD and for their expression. Actinobacillus suis contains both apxICABD(var.suis) and apxIICA(var. suis) operons and was shown to produce ApxI and ApxII toxin. Actinobacillus rossii contained the operons apxIICA(var.rossii) and apxIIICABD(var.rossii). However, only the toxin ApxII and not ApxIII could be detected in cultures of A. rossii. The Apx toxins found in A. suis and A. rossi may play a role in virulence of these pathogens. Actinobacillus lignieresii, which was included since it is phylogenetically very closely related to A. pleuropneumoniae, was found to contain a full apxICABD(var.lign.) operon which however lacks the -35 and -10 boxes in the promoter sequences. As expected from these results, no expression of ApxI was detected in A. lignieresii grown under standard culture conditions. Actinobacillus seminis, Actinobacillus equuli, Pasteurella aerogenes, Pasteurella multocida, Haemophilus parasuis, and also Mannheimia (Pasteurella) haemolytica, which is known to secrete leukotoxin, were all shown to be devoid of any of the apx toxin genes and did not produce ApxI, ApxII or ApxIII toxin proteins. However, proteins of slightly lower molecular mass than ApxI, ApxII and ApxIII which showed limited cross-reactions with monospecific, polyclonal anti-ApxI, anti-ApxII and anti-ApxIII were detected on immunoblot analysis of A. equuli, A. seminis and P. aerogenes. The presence of Apx toxins and proteins that imunologically cross react with Apx toxins in porcine Actinobacillus species other than A. pleuropneumoniae can be expected to interfere with serodiagnosis of porcine pleuropneumonia.

Pathophysiologic correlates of acute porcine pleuropneumonia

OBJECTIVE

To develop and evaluate an in vivo model to study early events in the pathogenesis of acute porcine pleuropneumonia.

ANIMALS

Thirty-six 6- to 8-week-old pigs.

PROCEDURE

Pigs were inoculated intranasally or endotracheally with Actinobacillus pleuropneumoniae; inoculation routes were compared by evaluation of clinical signs, gross and microscopic lung lesions, hematologic changes, serum zinc, iron, and haptoglobin concentrations, and inflammatory cytokines.

RESULTS

The 2 inoculation routes resulted in similar findings, although intranasal inoculation caused unilateral gross lung lesions, whereas endotracheal inoculation caused bilateral gross lesions. Clinical signs of disease were observed < 2 hours after endotracheal inoculation and 6 to 8 hours after intranasal inoculation. Total WBC counts did not differ significantly after inoculation by either inoculation route, although band neutrophils increased significantly. The earliest findings associated with A pleuropneumoniae inoculation, irrespective of route, were decreased serum zinc and iron concentrations. Serum haptoglobin concentrations were significantly increased after inoculation. Inoculation induced rapid influx of macrophages into the lung and local induction of proinflammatory cytokines. Northern blot analysis of total RNA from lung tissue indicated that inoculated pigs had increased concentrations of interleukin (IL)-1beta, IL-1alpha, and IL-8; tumor necrosis factor messenger RNA concentration was not increased.

CONCLUSIONS

Endotracheal inoculation with A pleuropneumoniae rapidly and consistently induced diffuse bilateral pneumonia; thus, this method may be useful for the study of acute pathophysiologic changes associated with bacterial pneumonia and may provide an experimental model for testing modalities for prevention and treatment of this and other respiratory tract diseases of pigs.

Porcine lung surfactant protein D: complementary DNA cloning, chromosomal localization, and tissue distribution

Porcine organs and lung surfactant have medically important applications in both xenotransplantation and therapy. We have started to characterize porcine lung surfactant by cloning the cDNA of porcine surfactant protein D (SP-D). SP-D and SP-A are important mediators in innate immune defense for the lung and possibly other mucosal surfaces. Porcine SP-D will also be an important reagent for use in existing porcine animal models for human lung infections. The complete cDNA sequence of porcine SP-D, including the 5' and 3' untranslated regions, was determined from two overlapping bacteriophage clones and by PCR cloning. Three unique features were revealed from the porcine sequence in comparison to SP-D from other previously characterized species, making porcine SP-D an intriguing species addition to the SP-D/collectin family. The collagen region contains an extra cysteine residue, which may have important structural consequences. The other two differences, a potential glycosylation site and an insertion of three amino acids, lie in the loop regions of the carbohydrate recognition domain, close to the carbohydrate binding region and thus may have functional implications. These variations were ruled out as polymorphisms or mutations by confirming the sequence at the genomic level in four different pig breeds. Porcine SP-D was shown to localize primarily to the lung and with less abundance to the duodenum, jejunum, and ileum. The genes for SP-D and SP-A were also shown to colocalize to a region of porcine chromosome 14 that is syntenic with the human and murine collectin loci.

Actinobacillus species and their role in animal disease

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

Early in vivo interactions of Actinobacillus pleuropneumoniae with tonsils of pigs

Twenty gnotobiotic piglets were inoculated with 5 x 10(8) colony forming units of an Actinobacillus pleuropneumoniae biotype 1-serotype 9 strain onto their tonsils. Five other piglets (controls) were inoculated with phosphate-buffered saline solution. Pigs were euthanized at 30 min, 90 min, 180 min, 6 h, 9 h, 12 h or 24 h after inoculation. At necropsy, samples were taken from the tonsils for bacteriological, histological, immuno-histochemical and electron microscopical examination. A. pleuropneumoniae was isolated from tonsils of all the infected pigs, but not from tonsils of the control pigs. Early after inoculation bacteria were mainly associated with the stratified squamous epithelium and detached epithelial cells. Vacuolization and desquamation of the epithelium was observed and many transmigrating neutrophils were present. At later times after inoculation, bacteria were found closely associated with the crypt-walls and with detached cells present in the crypts. A strong neutrophil migration was observed mainly in the deeper parts of the crypts. It is concluded that attachment of A. pleuropneumoniae to tonsillar epithelial cells probably constitutes a first step in establishing bacteria at this body site.

Molecular cloning and sequencing of the aroA gene from Actinobacillus pleuropneumoniae and its use in a PCR assay for rapid identification

The gene (aroA) of Actinobacillus pleuropneumoniae, serotype 2, encoding 5-enolpyruvylshikimate-3-phosphate synthase was cloned by complementation of the aroA mutation in Escherichia coli K-12 strain AB2829, and the nucleotide sequence was determined. A pair of primers from the 5' and 3' termini were selected to be the basis for development of a specific PCR assay. A DNA fragment of 1,025 bp was amplified from lysed A. pleuropneumoniae serotypes 1 to 12 of biovar 1 or from isolated DNA. No PCR products were detected when chromosomal DNAs from other genera were used as target DNAs; however, a 1,025-bp DNA fragment was amplified when Actinobacillus equuli chromosomal DNA was used as a target, which could be easily differentiated by its NAD independence. The PCR assay developed was very sensitive, with lower detection limits of 12 CFU with A. pleuropneumoniae cells and 0.8 pg with extracted DNA. Specificity and sensitivity make this PCR assay a useful method for the rapid identification and diagnosis of A. pleuropneumoniae infections.

Immunogenicity of Actinobacillus ApxIA toxin epitopes fused to the E. coli heat-labile enterotoxin B subunit

Peptides KDYGASTGSSL (Epil). SLLRRRRNGEDVSV (Epi3) and DDEIYGNDGHP (Epi6), predicted to constitute immunogenic epitopes of the hemolysin-cytotoxin ApxIA of Actinobacillus pleuropneumoniae were inserted into a surface-exposed loop of the B subunit of the E. coli heat-labile enterotoxin (EtxB). The resulting chimeric proteins were recognized by monospecific antibodies against purified native ApxI and by convalescent sera of pigs that were positive for A. pleuropneumoniae serotype 1. Mice anti-sera against chimeric proteins EtxB::ApxIAEpi3 and EtxB::ApxIAEpi6 reacted with purified ApxI. These results indicate that Epi3 and Epi6 regions constitute linear epitopes of the structural ApxIA protein toxin. Epitope Epi6 which is located in the structure of the glycine rich repeats in ApxI elicits the formation of hemolysin neutralizing antibodies when introduced into mice in the form of a chimeric EtxB fusion protein. We suggest that fusion of peptide sequences to EtxB is a useful tool for the analysis of epitopes of complex proteins such as RTX toxins.