Biochemical characterization of an antigenic saline extract of Actinobacillus pleuropneumoniae serotype 5 and identification of a serotype-specific antigen for ELISA serodiagnosis

A generic protocol to immobilize lipopolysaccharides on microbeads for multiplex analysis

Bead-based multiplex serodiagnostics enables simultaneous analysis of antibodies against several antigens. Binding of the antigens onto the surface of the bead, preserving the antigenicity of the antigen is a pivotal step to ensure high sensitivity and selectivity of the assay. Here, a generic method for immobilization of lipopolysaccharide (LPS) antigens from different Gram-negative bacteria to microbeads using non-covalent conjugation has been developed and tested. The method involves coupling of N,N-diethylethylenediamine (DEDA) and derivatives to microbeads. This enhances non-covalent interactions so that LPS is easily immobilized. LPS antigens from the Gram-negative bacteria Actinobacillus pleuropneumoniae (APP) and Salmonella enterica serogroup B (Sal. B) were immobilized on the DEDA-coupled microbeads. In parallel, the same LPS antigens were coupled to beads using two previously reported methods. The performance of microbeads coupled with antigen using the different methods was compared by measuring antibodies in positive and negative serum samples from pigs. DEDA-beads coupled with LPS detected pathogen specific serum antibodies with equal or higher sensitivity and specificity compared to the other coupling methods used in this study. Furthermore, derivatives of DEDA, where the tertiary amine was alkylated with a methyl (m-DEDA) and ethyl group (e-DEDA) to give a positively charged tetraalkylammonium group, were compared with DEDA for the binding of LPS antigens. Here, it was concluded that the DEDA-modified bead was most efficient in the binding of LPS antigens from two Actinobacillus pleuropneumoniae serovars and Salmonella enterica serogroup B.

Isolation of an atypical strain of Actinobacillus pleuropneumoniae serotype 1 with a truncated lipopolysaccharide outer core and no O-antigen

A field isolate of Actinobacillus pleuropneumoniae, the causative agent of porcine fibrinohemorrhagic necrotizing pleuropneumonia, was sent to the diagnostic laboratory for serotyping. The isolate presented a clear reaction, with both polyclonal antibodies against serotype 1 and monoclonal antibodies against the capsular polysaccharide of serotype 1. It also exhibited a PCR profile of Apx toxins expected for serotype 1. The isolate, however, failed to react with monoclonal antibodies against the O-antigen of serotype 1 lipopolysaccharide (LPS), suggesting a rough phenotype. The lipid A-core region of the isolate migrated faster than the corresponding region of the serotype 1 reference strain S4074 by Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting the presence of a truncated core. Sugar analysis and mass spectrometry analysis of the O-deacylated LPS from the field isolate were consistent with the absence of O-antigen and truncation of the outer core compared to the wild-type reference strain. Experimental infection of pigs confirmed the virulence of the isolate. This is the first report of an isolate of A. pleuropneumoniae serotype 1 with a truncated outer core and a rough LPS phenotype. Veterinary diagnostic laboratories should be vigilant, since infections caused by such an isolate will not be detected by serological tests based on LPS O-antigen.

Evaluation of an enzyme-linked immunosorbent assay for serological surveillance of infection with Actinobacillus pleuropneumoniae serotype 5 in pig herds

An indirect enzyme-linked immunoassay for serological surveillance of infection of pigs with Actinobacillus pleuropneumoniae (Ap) serotype 5 was developed. The antigen used was prepared from Ap serotype 5b strain L20. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that the antigen contained high molecular weight lipopolysaccharide (LPS) and presumably also capsular polysaccharide (CP). The Ap serotype 5 ELISA was tested using sera from pigs experimentally infected with the 12 different Ap serotypes of biotype 1 and with sera from herds naturally infected with Ap serotypes 5, 6, 7 and 12. Cross-reactions were shown in one pig from a herd naturally infected with Ap serotype 7 and in one pig from a herd naturally infected with Ap serotype 12. The herd sensitivities of the Ap5 ELISA and a complement fixation test (CFT) were both estimated to 1.0, on the basis of serum samples from six herds naturally infected with Ap serotype 5. The herd specificities of both tests were estimated to 0.98, based on serum samples from 123 pig herds (10 samples from each herd) from the Danish specific pathogen-free (SPF) programme for pig production.

Blocking enzyme-linked immunosorbent assay for detection of antibodies against Actinobacillus pleuropneumoniae serotype 6 in pig serum

A blocking enzyme-linked immunosorbent assay (ELISA) detecting antibodies against Actinobacillus pleuropneumoniae (Ap) serotype 6 was developed. The blocking ELISA was based on the inhibition of a polyclonal antibody raised against Ap serotype 6. Purified lipopolysaccharide from Ap serotype 6 was used as antigen. The blocking ELISA was tested against sera from pigs experimentally infected with the 12 serotypes of Ap biotype 1. Cross-reaction with serotypes 3 and 8 but not with other serotypes was observed. The sensitivity and specificity of the test on a herd level were evaluated with sera from herds naturally infected with serotypes 2, 6, 8 or 12 and with sera from herds free of infection with any Ap serotype. The blocking ELISA showed a high herd sensitivity (1.00 (0.79-1.00)) and specificity (0.97 (0.93-0.99)).

Actinobacillus pleuropneumoniae surface polysaccharides: their role in diagnosis and immunogenicity

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

Identification of Actinobacillus pleuropneumoniae strains of serotypes 7 and 4 using monoclonal antibodies: demonstration of common LPS O-chain epitopes with Actinobacillus lignieresii

Monoclonal antibodies (Mabs) against Actinobacillus pleuropneumoniae serotype 7 were produced and characterized. Three Mabs directed against surface polysaccharides were selected. One of the Mabs was directed against a capsular polysaccharide epitope (CPS) of A. pleuropneumoniae serotype 7 whereas two other Mabs reacted with different epitopes of the LPS O-chain. One of the latter reacted with the reference strain of serotype 7 and the other one with serotypes 7 and 4. These three Mabs were used to test, by Dot-ELISA, 508 field strains of A. pleuropneumoniae. None of the strains belonging to other serotypes different from serotypes 4 and 7 were positive with the Mabs. Used in combination, the CPS and one of the LPS O-chain directed Mabs were shown to be suitable for serotyping since they detected 100% of serotype 7 strains. In this study, we confirm for the first time that A. pleuropneumoniae serotype 4 is present in North America. Finally, both O-chain specific Mabs also reacted with the O-chain of Actinobacillus lignieresii. The cross-reactivity between the two species was confirmed using sera from pigs experimentally infected with A. pleuropneumoniae serotype 7 and A. lignieresii, using immunoblotting and ELISA. This is the first report of a specific cross-reactivity between the LPS of these bacterial species.

Characterization and protective activity of a monoclonal antibody against a capsular epitope shared by Streptococcus suis serotypes 1, 2 and 1/2

A monoclonal antibody (mAb Z3) was produced using BALB/c mice immunized with whole cells of Streptococcus suis serotype 2 reference strain S735. Screening by dot-ELISA showed that mAb Z3, of isotype IgG2b, reacted only with reference strains and field isolates of S. suis serotypes 1, 2 and 1/2. The recognized epitope was demonstrated to be polysaccharide in nature by periodate oxidation, and located in the capsule, since mAb Z3 reacted with purified capsular material by immunoblotting and was able to stabilize the capsule as shown by electron microscopy. Further characterization indicated that mAb Z3 may react specifically with the sialic acid moiety of the capsule, a common constituent of the polysaccharidic capsular material of the three capsular types, since sialidase-treated cells did not react with mAb Z3 in immunoblotting or indirect ELISA. Purified mAb Z3 was shown to significantly increase the rate of phagocytosis of S. suis cells by porcine monocytes and to activate the clearance of bacteria from the circulation in experimentally infected mice. However, mAb Z3 only offered partial protection to mice challenged with a minimal lethal dose. Thus, even though the capsule of S. suis seems to be an important virulence factor, the epitope recognized by mAb Z3 does not appear to be involved in complete protection against infection.

Evaluation of protective efficacy of an Actinobacillus pleuropneumoniae serotype 1 lipopolysaccharide-protein conjugate in mice

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia. The major adhesin of A. pleuropneumoniae has previously been identified as a lipopolysaccharide (LPS), and more recently, we demonstrated that high molecular mass LPS were involved in A. pleuropneumoniae adherence to porcine respiratory tract cells. We postulated that immunization with a LPS-based vaccine may confer a protective immunity. The high molecular mass O-polysaccharides obtained after acid hydrolysis and chromatographic separation were conjugated to bovine serum albumin (BSA) as a protein carrier. Groups of mice were injected twice with the following antigen preparations: whole-cell preparation, outer membrane preparation, O-polysaccharide-BSA conjugate, hydrolyzed LPS and phenol/water extracted LPS. A combination of different adjuvants was also used during these immunization procedures to induce a stronger immunological response to the polysaccharide antigen. Two weeks after the second injection, the mice were challenged intranasally with either homologous A. pleuropneumoniae serotype 1 strain or a serotype 5 strain. The highest survival rate, up to 80%, compared to the control groups (P < 0.05), was recorded when the mice were injected twice with 15 micrograms of carbohydrates of O-polysaccharide-BSA conjugate mixed with the saponin-derived adjuvant Quil A. Survival rates of between 60 and 70%, twice those observed in the control groups immunized with PBS, were recorded in mice injected with the O-polysaccharide-BSA conjugate mixed with other adjuvant preparations such as alhydrogel, peanut oil and Freund's incomplete adjuvant. However, the protection induced by the conjugate antigen preparation was serotype specific, because mice challenged with a serotype 5 strain were killed. Taken together, these results confirm the important role of A. pleuropneumoniae LPS in pathogenesis.

Identification of two core types in lipopolysaccharides of Actinobacillus pleuropneumoniae representing serotypes 1 to 12

Lipopolysaccharides (LPS) of Actinobacillus pleuropneumoniae were separated by Tricine-SDS-polyacrylamide gel electrophoresis, which has been shown to improve resolution of low-molecular-mass fast migrating bands. Strains representing the 12 serotypes of A. pleuropneumoniae can be divided in two groups according to the gel mobility of the core - lipid A region of their LPS. The first electromorphic core type (core type I), found in serotypes 1, 6, 9, and 11, had a migration slower than Salmonella typhimurium Ra LPS. The second electromorphic core type (core type II), found in the remaining serotypes (i.e., 2, 3, 4, 5, 7, 8, 10, and 12) had a migration similar to S. typhimurium Ra LPS. Furthermore, we observed that these two core types were antigenically different. Western blot analyses indicated that core - lipid A region of LPS from electromorphic core type I strains reacted when probed with serum from a pig experimentally infected with a core type I strain but not when probed with serum from a pig experimentally infected with a core type II strain. Conversely, core - lipid A region of LPS from electromorphic core type II strains reacted only when probed with serum from a pig experimentally infected with a core type II strain. Our results, based on both electrophoretic mobility and antigenicity, suggest the presence of two LPS core types in A. pleuropneumoniae.

Production and characterization of monoclonal antibodies against Actinobacillus pleuropneumoniae serotype 1

Three monoclonal antibodies (mAbs) against Actinobacillus pleuropneumoniae serotype 1, designated 4.2 A11 B5, 5.1 G8 F10 and 1.5 C5 F4 (IgG3, IgG2b and IgM respectively), were produced and characterized. mAbs 4.2 A11 B5 and 5.1 G8 F10 were directed against different epitopes located in the O chain of the LPS. Both clones also recognized reference strains of A. pleuropneumoniae serotypes 9 and 11. The mAb 1.5 C5 F4 reacted with the reference strain of A. pleuropneumoniae serotype 1, with the encapsulated strain 4045 (but not with its non-capsulated mutant) and with A. pleuropneumoniae serotype 1 purified capsular polysaccharides (CPS). The epitope was sensitive to periodate oxidation, heat-labile, and located in the capsular material of A. pleuropneumoniae serotype 1, as demonstrated by immunoblotting. Treatment of the CPS with 5% ammonium hydroxide eliminated the reaction, which may indicate that the epitope recognized by 1.5 C5 F4 mAb is a O-acetyl containing determinant. When different A. pleuropneumoniae field strains were tested, the percentage of strains recognized by the mAbs varied with the mAb and the test used. Cross-reactions associated with the LPS of some A. pleuropneumoniae serotype 5 field strains could be observed with the 4.2 A11 B5 mAb. Of the three mAbs characterized, 1.5 C5 F4 seemed to be the most suitable for A. pleuropneumoniae serotype 1 detection since it reacted with 99% of serotype 1 field strains and it did not recognize any of the strains belonging to other serotypes.

Characterization of monoclonal antibodies to O-antigen of lipopolysaccharide of Actinobacillus pleuropneumoniae serotype 2 and their use in the classification of field isolates

Monoclonal antibodies (mAbs) against Actinobacillus pleuropneumoniae serotype 2 (reference strain Shope 4226 and field isolate F46) were produced. Twelve hybridoma clones were selected against both strains, and all the antibodies secreted were found to be reactive with whole-cell antigen of the homologous strain in ELISA, whereas only one mAb was reactive in slide agglutination test. The predominant antibody classes were IgG2b and IgG3, although IgG1 and IgM were also obtained. Immunoblot assay showed that mAbs could recognize a ladder band profile which is in accordance with the O-antigen of lipopolysaccharide. Most of the epitopes involved were resistant to proteinase K and also to boiling in the presence of sodium dodecyl sulfate and reducing conditions, but they were sensitive to periodic acid. The 12 mAbs recognized neither reference strains of the remaining A. pleuropneumoniae serotypes nor other taxonomically related Gram-negative organisms. The suitability of mAbs for serotyping of field isolates was also examined, and a high correlation (97.4%) was found between the results previously established by indirect hemagglutination with polyclonal rabbit sera and those obtained by ELISA with mAbs. The panel of mAbs described in this study was found to be extremely useful for identifying field isolates belonging to serotype 2 and could be used as a complementary serotyping method.

Evaluation of a saline boiled extract, capsular polysaccharides and long-chain lipopolysaccharides of Actinobacillus pleuropneumoniae serotype 1 as antigens for the serodiagnosis of swine pleuropneumonia

A saline boiled extract (SBE), capsular polysaccharides (CPS) and long-chain lipopolysaccharides (LC-LPS) of Actinobacillus pleuropneumoniae serotype 1 have been evaluated in ELISA for the serodiagnosis of swine pleuropneumonia caused by this serotype. Mean optical densities (ODs) obtained with the three antigens using sera from negative herds as well as from animals experimentally and naturally exposed to A. pleuropneumoniae serotypes 1, 9 or 11 were not significantly different. The positive ELISA reaction with anti-serotypes 9 and 11 was unexpected with the CPS, which are supposed to be serotype-specific; LPS, and to a lesser extent proteins, were present in the CPS and appeared to be responsible for this reaction. In addition, sera from animals exposed to a field strain of A. pleuropneumoniae serotype 3 and to Actinobacillus suis presented a significantly lower mean OD (P < 0.001) when LC-LPS were used. Cross-reacting antigens consisted mainly of LPS core-lipid A present in the SBE and CPS. The specificity and the sensitivity of the ELISA were evaluated using three different cut-off values (the OD plus two, three and four times the standard deviation or SD) obtained with 667 negative sera. The diagnostic sensitivity was of 81% with the three antigens and the different thresholds. The diagnostic specificity was of 84, 86 and 88% for the mean plus two, three and four times the SD respectively using the SBE and the CPS, while that obtained with the LC-LPS was of 96, 98 and 99% using the same thresholds. In conclusion, LC-LPS make an easily obtainable antigen and seem to retain the best specificity while minimizing losses of sensitivity.

Lipopolysaccharides of Actinobacillus pleuropneumoniae (serotype 1): a readily obtainable antigen for ELISA serodiagnosis of pig pleuropneumonia

A saline extract of boiled-formalinized whole cells of Actinobacillus pleuropneumoniae serotype 1 reference strain (Shope 4074) has been previously used as the antigen in an enzyme-linked immunosorbent assay (ELISA) for serodiagnosis of swine pleuropneumonia. Phenol extraction of this crude extract permitted the recovery of LPS with long O-chains in the aqueous phase. This antigen was shown to be specific for serotypes 1, 9 and 11 as they all possess structurally similar O-chains. Immunoblotting was used to identify the fraction present in the crude extract of strain 4074 responsible for cross-reactions observed in ELISA with a serum raised against a serotype 3 strain of A. pleuropneumoniae. The specific reactions in ELISA were shown to be associated with long O-chain LPS and the cross-reactions to LPS with short O-chains. LPS seem to be an important antigen for A. pleuropneumoniae serotype 1 as all homologous sera tested reacted with it. This antigen is easily recovered from the crude extract and it can be used in minute amounts (1-6 micrograms) for ELISA serodiagnosis of pig pleuropneumonia.

Evaluation of long chain lipopolysaccharides (LC-LPS) of Actinobacillus pleuropneumoniae serotype 5 for the serodiagnosis of swine pleuropneumonia

Long chain lipopolysaccharides (LC-LPS) of Actinobacillus pleuropneumoniae serotype 5 have been evaluated and compared with a crude boiled extract (CBE) in ELISA for the serodiagnosis of swine pleuropneumonia caused by this serotype. The mean optical density (OD) obtained with the LC-LPS in ELISA using sera from negative herds as well as from animals experimentally and naturally exposed to A. pleuropneumoniae serotype 5 was not significantly different from that obtained with the CBE. However, sera from animals exposed to serotypes of A. pleuropneumoniae other than serotype 5 presented a significantly lower mean OD (P < 0.05) when the LC-LPS was used. As a consequence, it was demonstrated that a high percentage of non-specific cross-reactions were eliminated, without losing specificity. The specificity and the sensitivity of the LC-LPS- and CBE-ELISA were evaluated using two different cut-off values (the OD plus two and three standard deviations) (SD) obtained from 593 sera from negative herds. The LC-LPS appeared a more suitable antigen than the CBE, since the sensitivity and the specificity (obtained with both thresholds) were statistically improved (P < 0.01). A threshold of 0.244 (mean OD plus three SD) for the LC-LPS-ELISA seemed more suitable, since a sensitivity of 79% and a specificity of 97% was achieved. Nevertheless, it may be advisable to keep a buffer range (OD between 0.194 and 0.243) and to consider sera presenting values within this range as suspicious. In the present study, the complement fixation test presented a high specificity (97%) and a very low sensitivity (47%). A herd with animals presenting ELISA positive and CFT negative results in serology, along with the absence of suggestive lesions should not be considered as a non-infected herd.