Indirect enzyme-linked immunosorbent assay for detection of antibody to a 110,000-molecular-weight hemolysin of Actinobacillus pleuropneumoniae

Simultaneous detection of antibodies against Apx toxins ApxI, ApxII, ApxIII, and ApxIV in pigs with known and unknown Actinobacillus pleuropneumoniae exposure using a multiplexing liquid array platform

Surveillance for the presence of Actinobacillus pleuropneumoniae infection in a population plays a central role in controlling the disease. In this study, a 4-plex fluorescent microbead-based immunoassay (FMIA), developed for the simultaneous detection of IgG antibodies to repeat-in-toxin (RTX) toxins (ApxI, ApxII, ApxIII, and ApxIV) of A. pleuropneumoniae, was evaluated using (i) blood serum samples from pigs experimentally infected with each of the 15 known A. pleuropneumoniae serovars or with Actinobacillus suis, (ii) blood serum samples from pigs vaccinated with a bacterin containing A. pleuropneumoniae serovar 1, 3, 5, or 7, and (iii) blood serum samples from pigs with an unknown A. pleuropneumoniae exposure status. The results were compared to those obtained in a previous study where a dual-plate complement fixation test (CFT) and three commercially available enzyme-linked immunosorbent assays (ELISAs) were conducted on the same sample set. On samples from experimentally infected pigs, the 4-plex Apx FMIA detected specific seroconversion to Apx toxins as early as 7 days postinfection in a total of 29 pigs inoculated with 14 of the 15 A. pleuropneumoniae serovars. Seroconversion to ApxII and ApxIII was detected by FMIA in pigs inoculated with A. suis. The vaccinated pigs showed poor humoral responses against ApxI, ApxII, ApxIII, and ApxIV. In the field samples, the humoral response to ApxIV and the A. pleuropneumoniae seroprevalence increased with age. This novel FMIA (with a sensitivity of 82.7% and a specificity of 100% for the anti-ApxIV antibody) was found to be more sensitive and accurate than current tests (sensitivities, 9.5 to 56%; specificity, 100%) and is potentially an improved tool for the surveillance of disease and for monitoring vaccination compliance.

Evaluation of diagnostic assays for the serological detection of Actinobacillus pleuropneumoniae on samples of known or unknown exposure

Accurate diagnosis of exposure to Actinobacillus pleuropneumoniae is important for maintaining negative farms. In the present study, the ability of a dual-plate complement fixation (CF) assay and 3 commercially available enzyme-linked immunosorbent assays (ELISAs; quad-plate ELISA-1, single-plate ELISA-2, and single-plate ELISA-3) in detecting serological evidence of A. pleuropneumoniae exposure was compared using serum samples of experimentally infected or vaccinated pigs, or field samples from the United States. Forty-two pigs were divided into groups of 2 pigs and were inoculated with 1 of 15 A. pleuropneumoniae strains representing all known serovars of A. pleuropneumoniae, or with Actinobacillus suis, or were vaccinated with a bacterin containing A. pleuropneumoniae serovar 1, 3, 5, or 7. Serum samples collected at the day of inoculation or vaccination and 7, 14, 21, and 28 days later were used to compare the assays. On samples from experimentally infected pigs, the dual-plate CF assay, quad-plate ELISA-1, single-plate ELISA-2, and single-plate ELISA-3 had sensitivities of 0.46, 0.74, 0.13, and 0.13 and specificities of 0.90, 1.0, 1.0, and 1.0, respectively. Vaccinated pigs were identified only by the dual-plate CF assay and the quad-plate ELISA-1. In addition, 90 serum samples with unknown A. pleuropneumoniae exposure collected under field conditions were tested with all assays. The agreement of the 4 assays on field samples was slight to fair. While several assays are available for demonstration of A. pleuropneumoniae exposure, differences in assay targets complicate test choices. Decisions on which assay or combination of assays to use depend on the specific reasons for running the assays.

Photonic biosensor assays to detect and distinguish subspecies of Francisella tularensis

The application of photonic biosensor assays to diagnose the category-A select agent Francisella tularensis was investigated. Both interferometric and long period fiber grating sensing structures were successfully demonstrated; both these sensors are capable of detecting the optical changes induced by either immunological binding or DNA hybridization. Detection was made possible by the attachment of DNA probes or immunoglobulins (IgG) directly to the fiber surface via layer-by-layer electrostatic self-assembly. An optical fiber biosensor was tested using a standard transmission mode long period fiber grating of length 15 mm and period 260 μm, and coated with the IgG fraction of antiserum to F. tularensis. The IgG was deposited onto the optical fiber surface in a nanostructured film, and the resulting refractive index change was measured using spectroscopic ellipsometry. The presence of F. tularensis was detected from the decrease of peak wavelength caused by binding of specific antigen. Detection and differentiation of F. tularensis subspecies tularensis (type A strain TI0902) and subspecies holarctica (type B strain LVS) was further accomplished using a single-mode multi-cavity fiber Fabry-Perot interferometric sensor. These sensors were prepared by depositing seven polymer bilayers onto the fiber tip followed by attaching one of two DNA probes: (a) a 101-bp probe from the yhhW gene unique to type-A strains, or (b) a 117-bp probe of the lpnA gene, common to both type-A and type-B strains. The yhhW probe was reactive with the type-A, but not the type-B strain. Probe lpnA was reactive with both type-A and type-B strains. Nanogram quantities of the target DNA could be detected, highlighting the sensitivity of this method for DNA detection without the use of PCR. The DNA probe reacted with 100% homologous target DNA, but did not react with sequences containing 2-bp mismatches, indicating the high specificity of the assay. These assays will fill an important void that exists for rapid, culture-free, and field-compatible diagnosis of F. tularensis.

Use of an inhibition enzyme-linked immunosorbent assay for quantification of capsular polysaccharide or proteins in vaccines

An inhibition enzyme-linked immunosorbent assay (ELISA) is described for quantification of capsular polysaccharide or proteins in vaccines and other samples containing whole cells or extracts of Actinobacillus pleuropneumoniae. The assay can be used to quantify any antigen that can be purified and for which highly specific antibodies are not available. The assay can be carried out by any laboratory capable of performing an ELISA.

Expression of the apxIV gene in pigs naturally infected with Actinobacillus pleuropneumoniae

The apxIV gene was detected, by in-situ hybridization with a non-radioactive digoxigenin-labelled probe, in formalin-fixed, paraffin wax-embedded samples of lung tissue from 10 pigs naturally infected with Actinobacillus pleuropneumoniae. A 442 base pair DNA probe of the apxIV gene from A. pleuropneumoniae serotype 2 was generated by the polymerase chain reaction. All 10 pigs infected with A. pleuropneumoniae serotypes 2, 5, 6, or an untypable strain showed a distinct, positive signal in the degenerate alveolar leucocytes in alveolar spaces, and in the dense zone of degenerated cells in granulation tissue surrounding the necrotic areas. Thus, the study demonstrated the presence of the apxIV gene in pleuropneumonic lesions caused by A. pleuropneumoniae.

Genotypic prevalence of apx1V in Actinobacillus pleuropneumoniae field isolates

A total of 90 Actinobacillus pleuropneumoniae field strains from pigs were serotyped by slide agglutination and analyzed for the presence of the apxIV gene by polymerase chain reaction. Of the 90 isolates serotyped, serotypes 2 (47 isolates) and 5 (25 isolates) were the most common, followed by serotype 6 (10 isolates). Three isolates belonged to serotype 7, and 5 isolates could not be typed. All 90 A. pleuropneumoniae field isolates tested carried the apxIV gene. This gene is species specific rather than serotype specific. Therefore, the ApxIV toxin has potential value for use both in vaccines and in diagnostic tests.

Evaluation of an indirect enzyme-linked immunosorbent assay (ELISA) for detection of antibodies to the Apx toxins of Actinobacillus pleuropneumoniae

The reference strains of the 12 serotypes of Actinobacillus pleuropneumoniae express one or two of three different RTX exotoxins designated Apx I, Apx II and Apx III. The toxins are important virulence factors. In the present study, ELISAs with purified Apx I, Apx II and Apx III, respectively, as antigen were evaluated as candidates for serological diagnosis of Actinobacillus pleuropneumoniae infection in pigs. The pigs were inoculated with biotype 1, serotypes 1-12, and biotype 2, serotype 14, respectively. A strong humoral antibody response was seen to all the three antigens in most pigs irrespective of the serotype used for inoculation. However, titers to the exotoxins secreted by the serotype used for inoculation were generally highest. The results show that toxin proteins of Actinobacillus pleuropneumoniae are antigenically related and that a correlation between serotype and secretion of exotoxin is not revealed serologically in the ELISA test.

Cloning and mutagenesis of a serotype-specific DNA region involved in encapsulation and virulence of Actinobacillus pleuropneumoniae serotype 5a: concomitant expression of serotype 5a and 1 capsular polysaccharides in recombinant A. pleuropneumoniae serotype 1

A DNA region involved in Actinobacillus pleuropneumoniae serotype 5 capsular polysaccharide (CP) biosynthesis was identified and characterized by using a probe specific for the cpxD gene involved in CP export. The adjacent serotype 5-specific CP biosynthesis region was cloned from a 5.8-kb BamHI fragment and an 8.0-kb EcoRI fragment of strain J45 genomic DNA. DNA sequence analysis demonstrated that this region contained four complete open reading frames, cps5A, cps5B, cps5C, and cps5D. Cps5A, Cps5B, and Cps5C showed low homology with several bacterial glycosyltransferases involved in the biosynthesis of lipopolysaccharide or CP. However, Cps5D had high homology with KdsA proteins (3-deoxy-D-manno-2-octulosonic acid 8-phosphate synthetase) from other gram-negative bacteria. The G+C content of cps5ABC was substantially lower (28%) than that of cps5D and the rest of the A. pleuropneumoniae chromosome (42%). A 2.1-kb deletion spanning the cloned cps5ABC open reading frames was constructed and transferred into the J45 chromosome by homologous recombination with a kanamycin resistance cassette to produce mutant J45-100. Multiplex PCR confirmed the deletion in this region of J45-100 DNA. J45-100 did not produce intracellular or extracellular CP, indicating that cps5A, cps5B, and/or cps5C were involved in CP biosynthesis. However, biosynthesis of the Apx toxins, lipopolysaccharide, and membrane proteins was unaffected by the mutation. Besides lack of CP biosynthesis, and in contrast to J45, J45-100 grew faster, was sensitive to killing in precolostral calf serum, and was avirulent in pigs at an intratracheal challenge dose three times the 50% lethal dose (LD50) of strain J45. At six times the J45 LD50, J45-100 caused mild to moderate lung lesions but not death. Electroporation of cps5ABC into A. pleuropneumoniae serotype 1 strain 4074 generated strain 4074(pJMLCPS5), which expressed both serotype 1 and serotype 5 CP. However, serotype 1 capsule expression was diminished in 4074(pJMLCPS5) in comparison to 4074. The recombinant strain produced significantly less total CP (serotypes 1 and 5 CP combined) in log phase (P = 0.0012) but significantly more total CP in late stationary phase than 4074 (P < 0.0001). In addition, strain 4074(pJMLCPS5) caused less mortality and bacteremia in pigs and mice following respiratory challenge than strain 4074, indicating that virulence was affected by diminished capsule production. These results emphasize the importance of CP in the serum resistance and virulence of A. pleuropneumoniae.

Detection of antibodies against Actinobacillus pleuropneumoniae serotype 5 using an inhibition enzyme immunoassay

An inhibition enzyme immunoassay (EIA) for detection of antibodies against A. pleuropneumoniae serotype 5 (App-5) in pig sera, based on the inhibition of the binding of an App-5 specific monoclonal antibody was established. The monoclonal antibody (MAb 210-F11) was found to be directed against an epitope on the O-chain of App-5 LPS. In the inhibition EIA, highly purified App-5 LPS was used to coat microtitre plates. Serial dilutions of pig sera were added to the plates prior to the addition of the MAb 210-F11. The degree of binding of App-5 antibodies from pig sera was determined as the percentage inhibition of the MAb 210-F11. Pig serum from specific pathogen free (SPF) herds, from experimentally infected animals, and from acutely and chronically infected herds were tested. A serum dilution of 1/30 was found to be optimal, when using 50% inhibition as the discriminating inhibition percentage. No cross-reactivity was observed with serum from pigs infected with other App serotypes or bacteria isolated from the respiratory tract, such as A. suis and H. parasuis. The inhibition EIA will be used for surveillance of App-5 antibodies in SPF and conventional herds.

Safety, efficacy, and immunogenicity of a recombinant Osp subunit canine Lyme disease vaccine

A subunit canine Lyme disease vaccine formulated with recombinant lipidated Osp A and OspB and saponin QS21 was assessed for safety, protective efficacy, and immunogenicity. Ten normal beagles were subcutaneously vaccinated twice at age 12 and 16 weeks, respectively. Three months after the second vaccination, the vaccinates and another 10 nonvaccinated control beagles were challenged by feeding ticks on each dog for 5 days using eight field-collected adult female and six adult male Ixodes scapularis infected with Lyme disease spirochetes per dog. Adverse reactions associated with the vaccinations were limited to injection site swellings which occurred within the first 48 h and resolved within a week. The local reaction was independent of vaccination times and tick challenge. On the basis of typical clinical signs, xenodiagnosis, and diagnostic immunoblotting, all 10 controls were infected; five developed lameness and three of them experienced at least two to three episodes of limping during a 10-month monitoring period. In contrast, eight of ten vaccinates were protected and two infected vaccinates, as judged by xenodiagnosis, were asymptomatic. None of the protected vaccinates developed antibodies to diagnostic spirochetal antigens other than OspA and OspB. In contrast, most controls produced antibodies to borrelial antigens, but not to OspA and OspB. Antibody production in vaccinates receiving a third vaccination 10 months postchallenge was greatly boosted; the geometric mean antibody titer was significantly higher (P < 0.0001) than that tested prechallenge. Thus, the subunit canine Lyme disease vaccine was safe and protective and elicited immunological memory. Vaccinated dogs were serologically distinguishable from those naturally exposed.

Production of Apx toxins by field strains of Actinobacillus pleuropneumoniae and Actinobacillus suis

The three Apx toxins of Actinobacillus pleuropneumoniae have potential value for use in vaccines and diagnostic tests which will be species specific instead of serotype specific, provided that the Apx toxins are species specific and all field strains produce these toxins. We examined 114 A. pleuropneumoniae field strains and found that they secreted either ApxI, ApxII, ApxI and ApxII, or ApxII and ApxIII and secreted no other cytolytic activities. However, proteins similar to ApxI and ApxII were also produced by Actinobacillus suis.

Detection of Actinobacillus pleuropneumoniae serotype 2 antibodies in pig sera by an inhibition enzyme immuno assay (EIA)

An inhibition Enzyme Immuno Assay (EIA) for detection of antibodies against A. pleuropneumoniae serotype 2 (App-2) in pig sera, based on the inhibition of the binding of an App-2 specific monoclonal antibody was established. The monoclonal antibody (mAb 102-G02) was found to be directed against an epitope on the O-chain of App-2 LPS. Some App-2 isolates did not react with the mAb 102-G02. These isolates are referred to as App-2X. In the inhibition EIA highly purified App-2 LPS ws used to coat microtiter plates. Serial dilutions of pig sera were added to the plates prior to the mAb 102-G02. The degree of binding of App-2 antibodies from pig sera was determined as the percentage inhibition of the mAb 102-G02. Pig sera from specific pathogen free (SPF) herds, from experimentally infected animals, and from conventionel herds were tested. A serum dilution of 1/200 was found to be optimal, when using 50% inhibition as the discriminating inhibition percentage. Serum from App-2X infected herds showed a lower reactivity as compared to serum from App-2 infected herds. No crossreactivity was observed with serum from pigs infected with other App serotypes. The sensitivity and specificity were 100% and 98.9%, respectively.

Safety, stability, and efficacy of noncapsulated mutants of Actinobacillus pleuropneumoniae for use in live vaccines

Clonal, noniridescent mutants of Actinobacillus pleuropneumoniae serotypes 1 and 5 were isolated following chemical mutagenesis with ethyl methanesulfonate. The absence of any detectable capsule was confirmed by inhibition radioimmunoassay. There were no differences between the parent and mutant strains in lipopolysaccharide or protein electrophoretic profiles or in hemolytic activity. There was no detectable reversion to the encapsulated phenotype in vitro after passage in mice or pigs or in microporous capsules that were implanted subcutaneously in pigs for 6 weeks. The mutants were able to survive for more than 1 week in pigs following subcutaneous inoculation, which resulted in a strong immune response to whole cells and Apx toxins I and II. Intratracheal challenge of pigs with the serotype 5 mutant at a dose 1 log greater than the 50% lethal dose for the parent resulted in no clinical disease or lesions except in one pig that had slight pneumonia and pleuritis. Twenty-four hours after challenge, A. pleuropneumoniae could not be recovered from the respiratory tracts of any of the challenged pigs except for the one infected pig; this isolate remained noncapsulated. Immunization of pigs with one or both serotypes of noncapsulated mutants protected all pigs against clinical disease following intratracheal challenge with the virulent homologous or heterologous serotype. Nonimmunized control pigs and pigs immunized with a commercial bacterin died or had to be euthanized within 24 h of challenge. Thus, live noncapsulated mutants of A. pleuropneumoniae may provide safe and cost-effective protection against swine pleuropneumonia. These observations support the possibility that noncapsulated mutants of other encapsulated, toxin-producing bacteria may also prove to be efficacious live-vaccine candidates.

Evaluation of an immunoperoxidase technique using an only biotin-labeled antibody for the demonstration of Actinobacillus pleuropneumoniae in tissue sections

In this report an immunoperoxidase technique (IPB) using an only biotin-labeled antibody is compared to culture isolation method (CIM) for the demonstration of Actinobacillus pleuropneumoniae serotype 1 in tissues of mice inoculated intravenously. The organism was isolated in 86.7% of mice and detected by IPB in 100% of cases. A. pleuropneumoniae antigen was mainly demonstrated in the liver, spleen and lungs, but also in the kidney and brain, being especially located in the cytoplasm of macrophages. Significant differences were observed between the results obtained by both tests (P < 0.001). Cross-reactions occurred by IPB when a rabbit anti-A. pleuropneumoniae serotype 11 serum was tested but never when an anti-serotype 9 serum was used. The immunoperoxidase test here described yielded much better results than CIM and it could be useful in routine diagnosis of A. pleuropneumoniae infections.

Serotype specificity of immunological assays for the capsular polymer of Actinobacillus pleuropneumoniae serotypes 1 and 9

The cross-reactivity of the purified polysaccharides of Actinobacillus pleuropneumoniae serotypes 1 and 9 were examined using a variety of highly sensitive assays, such as radioimmunoassay, latex agglutination, enzyme-linked immunosorbent assay (ELISA), and immunoblotting. In addition, conventional immunodiffusion was included for comparison. Latex agglutination, utilizing affinity-purified IgG to capsule, was also used to serotype whole cells. Agglutination or precipitation tests (radioimmunoassay, latex agglutination, and immunodiffusion) indicated no cross-reactivity between the capsules of serotypes 1 and 9, and no cross-reactivity between whole cells by latex agglutination. Assays that required binding of the capsule to a solid support (ELISA and immunoblotting) did demonstrate cross-reactions between serotypes 1 and 9 capsules, although reactions with the heterologous serotype were weaker than with the homologous serotype. The cross-reactivity could not be attributed solely to nonspecific factors because similar cross-reactivity did not occur with serotype 5 or 7 capsules by any assay. Reactivity of antisera with homologous or heterologous capsule was reduced, but not completely eliminated, by adsorption with washed, live bacteria of the heterologous serotype. Thus, the assay, as well as the antigen or specificity of the antibody reagent used, may influence the results of A. pleuropneumoniae serotyping or serological tests.

Identification of a second hemolysin (HlyII) in Actinobacillus pleuropneumoniae serotype 1 and expression of the gene in Escherichia coli

Hemolysin genes of the reference strains of Actinobacillus pleuropneumoniae serotypes 1 and 2 were identified, cloned, and expressed in Escherichia coli by using polymerase chain reaction amplification with oligonucleotides derived from the DNA sequence of the corresponding appA gene from A. pleuropneumoniae serotype 5. The three genes from serotypes 1, 2, and 5 have identical restriction maps and appear to encode a hemolysin which was previously identified in serotype 2 and designated HlyII. Gene appA is different from hlyIA encoding the major hemolysin type I (HlyI) which was identified earlier in serotype 1. Polymerase chain reaction amplification with oligonucleotides derived from the DNA sequence of hlyIA of serotype 1 showed that the gene encoding HlyI is present in serotype 1 but not in serotype 2, in contrast to the gene encoding HlyII that was present in both serotypes. This was confirmed by Western blot (immunoblot) experiments using monoclonal antibodies specific for either recombinant HlyI or recombinant HlyII, which showed that A. pleuropneumoniae serotype 1 strain 4074 produces both HlyI and HlyII, whereas serotype 2 strain S1536 produces only HlyII. The expression of both hemolysins was investigated in all serotypes by the use of monoclonal antibodies. HlyI was shown to be expressed by the reference strains of serotypes 1, 5a, 5b, 9, 10, and 11, whereas HlyII was shown to be expressed by the reference strains of all 12 serotypes tested except serotype 10. A. pleuropneumoniae serotype 1 strain 4074 is the first bacterium which has been shown to contain two different actively expressed RTX toxin genes. Comparison of our data with those from other groups shows that the originally described strongly hemolytic hemolysin type I (HlyI) corresponds to cytolysin I (ClyI) which was recently described by others, while the weakly hemolytic hemolysin type II (HlyII) seems to be identical to ClyII and AppA.

Viability of Actinobacillus pleuropneumoniae in frozen pig lung samples and comparison of different methods of direct diagnosis in fresh samples

A comparative study on different methods of diagnosis of Actinobacillus pleuropneumoniae from both fresh and frozen pig lungs is described. A total of 196 lung tissues with pneumonic lesions were examined for culture isolation on chocolate blood agar, as well as for antigen detection by means of the coagglutination test, the immunodiffusion test and the indirect ELISA. These samples were subsequently frozen for 1 yr and then they were recultured. A. pleuropneumoniae was recovered from fresh lung specimens in 30 cases (15.3%) and from frozen samples in only two cases (0.9%). Such a different degree of isolation demonstrates that long freezing had an adverse effect on the viability of this organism in lung samples. A pleuropneumoniae detection was positive in 134 samples (68.4%) by at least one of the immunological techniques examined. The indirect ELISA was the most sensitive and specific test, with antigen detected in 125 lungs (63.8%). In comparison with the coagglutination and immunodiffusion tests, the sensitivities of the indirect ELISA were 95.8 and 93.7%, and the specificities were 67.0 and 63.4%, respectively.

Nucleotide sequence of the hemolysin I gene from Actinobacillus pleuropneumoniae

The DNA sequence of the gene encoding the structural protein of hemolysin I (HlyI) of Actinobacillus pleuropneumoniae serotype 1 strain 4074 was analyzed. The nucleotide sequence shows a 3,072-bp reading frame encoding a protein of 1,023 amino acids with a calculated molecular size of 110.1 kDa. This corresponds to the HlyI protein, which has an apparent molecular size on sodium dodecyl sulfate gels of 105 kDa. The structure of the protein derived from the DNA sequence shows three hydrophobic regions in the N-terminal part of the protein, 13 glycine-rich domains in the second half of the protein, and a hydrophilic C-terminal area, all of which are typical of the cytotoxins of the RTX (repeats in the structural toxin) toxin family. The derived amino acid sequence of HlyI shows 42% homology with the hemolysin of A. pleuropneumoniae serotype 5, 41% homology with the leukotoxin of Pasteurella haemolytica, and 56% homology with the Escherichia coli alpha-hemolysin. The 13 glycine-rich repeats and three hydrophobic areas of the HlyI sequence show more similarity to the E. coli alpha-hemolysin than to either the A. pleuropneumoniae serotype 5 hemolysin or the leukotoxin (while the last two are more similar to each other). Two types of RTX hemolysins therefore seem to be present in A. pleuropneumoniae, one (HlyI) resembling the alpha-hemolysin and a second more closely related to the leukotoxin. Ca(2+)-binding experiments using HlyI and recombinant A. pleuropneumoniae prohemolysin (HlyIA) that was produced in E. coli shows that HlyI binds 45Ca2+, probably because of the 13 glycine-rich repeated domains. Activation of the prohemolysin is not required for Ca2+ binding.

Identification of hemolytic and cytotoxic proteins of Actinobacillus pleuropneumoniae by use of monoclonal antibodies

Monoclonal antibodies (MAbs) were raised against extracellular hemolytic and cytotoxic proteins of Actinobacillus pleuropneumoniae serotypes 2 and 9. MAbs were tested for inhibition of hemolytic and cytotoxic activities of the reference strains of A. pleuropneumoniae serotypes 1 to 12. Five MAbs inhibited hemolytic or cytotoxic activities of one or more serotypes and were used to probe Western blots (immunoblots) of cytotoxic and hemolytic culture filtrates of serotypes 1 to 12. The MAbs recognized three different proteins: (i) a protein of approximately 103 kDa that was associated with a weak hemolytic activity and a moderate cytotoxic activity, (ii) a protein of approximately 105 kDa that was associated with a strong hemolytic activity and a strong cytotoxic activity, and (iii) a protein of approximately 120 kDa that was associated with a strong cytotoxic activity but not with hemolytic activity. Serotypes 6, 7, and 12 produced only the 103-kDa protein, and serotype 10 produced only the 105-kDa protein. The other serotypes produced two proteins: serotypes 1, 5, 9, and 11 produced the 103- and 105-kDa proteins, and serotypes 2, 3, 4, and 8 produced the 103- and 120-kDa proteins.

Immunological properties of Actinobacillus pleuropneumoniae hemolysin I

The 105 kDa hemolysin I protein from Actinobacillus pleuropneumoniae serotype I type strain 4074 (HlyI) was shown by immunoblot analysis to be the predominant immunogenic protein if convalescent field sera or sera from pigs experimentally infected with A. pleuropneumoniae serotype 1 were used. SDS gel- and immunoblot-analysis using total culture, washed cells or culture supernatant showed that HlyI is essentially secreted and is not found attached to the bacteria. Proteins in the 105 kDa range that react strongly with anti-HlyI antibody, are produced by all serotypes and are presumed to be their hemolysins. Sera from pigs experimentally infected with each of the 12 serotypes strongly reacted with HlyI. In addition, some sera from pigs that were confirmed to be negative for A. pleuropneumoniae, also reacted with HlyI as well as with related proteins from Actinobacillus rossii and Actinobacillus suis. These two species produce proteins in the 105 kDa range which cross-react strongly with HlyI. They could be the source of the immunological reactions of the A. pleuropneumoniae-negative sera with HlyI. However, no cross-reactions could be found between HlyI and the Pasteurella haemolytica leukotoxin, the Escherichia coli alpha-hemolysin or related proteins from various hemolytic E. coli strains isolated from pigs. The immunological cross-reactions of HlyI with related proteins from A. rossii, A. suis and possibly from other bacterial species may create uncertainty in interpretation if HlyI is used as the antigen in serodiagnosis of A. pleuropneumoniae.

Construction of a DNA probe and detection of Actinobacillus pleuropneumoniae by using polymerase chain reaction

A 1.5-kb Actinobacillus pleuropneumoniae 4074 DNA fragment from a genomic library was found to hybridization. No cross-hybridization hybridization. No cross-hybridization was detected with DNAs from hemolytic members of the family Pasteurellaceae. From the nucleotide sequence of the putative genomic probe, three primers were synthesized for use in polymerase chain reactions (PCRs), with 31 strains tested by using purified and crude DNA targets. PCR amplification products of 610 and 985 bp were observed in nucleic acids extracted from the 12 known serotypes and a biotype 2 strain. Template DNAs from other gram-negative and gram-positive bacteria, some of them found in the normal flora of swine and the upper respiratory tract, were not amplified by PCR. The only exception was an amplification of a similar 610- or 985-bp sequence in Actinobacillus lignieresii, a species that is closely related to A. pleuropneumoniae but that has never been isolated from swine. Amplification of specific A. pleuropneumoniae sequences by PCR directly from clinical specimens may find applications in the identification of asymptomatic carriers as well as in efforts to eradicate porcine pleuropneumonia.

Characterization of a non-hemolytic mutant of Actinobacillus pleuropneumoniae serotype 5: role of the 110 kilodalton hemolysin in virulence and immunoprotection

To determine the role of hemolysin(s) in virulence and immunoprotection, non-hemolytic mutants of Actinobacillus pleuropneumoniae serotype 5, strain J45, were isolated following chemical mutagenesis. One mutant was selected for extensive characterization. Differences in capsule content, or in lipopolysaccharide or membrane protein electrophoretic profiles of the parent and mutant were not detected. A predominant, calcium-inducible protein of 110 kDa was present in culture supernatant of the parent, but absent from the mutant. Two-dimensional (2-D) gel electrophoresis confirmed that the 110 kDa protein was absent in culture supernatant of the mutant, but few, if any, minor differences could be detected in whole-cell proteins between the parent and mutant. The mutant totally lacked extracellular hemolytic and cytotoxic activity. Lysates of whole cells of the mutant contained weak hemolytic activity, and the 110 kDa protein could be detected by immunoblotting. Neutralization titers were negative in pigs immunized with the mutant or purified, denatured hemolysin, although enzyme-immunoassay titers were detected. Four additional independently isolated non-hemolytic mutants were avirulent in pigs and mice at doses greater than 10 times the lethal dose of the parent. Neither pigs nor mice were protected against lethal infection following immunization with the non-hemolytic mutant. We conclude that the 110 kDa hemolysin plays an important role in bacterial virulence and the pathogenesis of pleuropneumonia, and that sufficiently high levels of neutralizing antibodies to the 110 kDa hemolysin may be required for protection of pigs against disease.