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

EFFECT OF BACTERIAL AGENTS OF PORCINE RESPIRATORY DISEASE COMPLEX ON PRODUCTIVE INDICES AND SLAUGHTER WEIGHT

Abstract Porcine respiratory disease complex comprises the interaction of two or more infectious agents. The major bacterial agents involved were investigated in 115 finishing pigs at a farm in São Paulo State, Brazil: Actinobacillus pleuropneumoniae (serology, bacterial culture, and multiplex PCR), Mycoplasma hyopneumoniae (Mhyo) (nested PCR), Pasteurella multocida (multiplex PCR), Haemophilus parasuis (PCR multiplex), and Streptococcus sp. (bacterial culture). Macroscopic and microscopic lung lesions were evaluated, and zootechnical indices were recorded. Mhyo occurred in 113 animals (98.3%), seventeen of which were co-infected with Streptococcus sp. The finding of emphysematous lung was associated with significantly lower final and carcass weight at slaughter. Although vaccinated against Mhyo with an inactivated immunogen, almost 100% of the animals were infected. Mhyo infection with and without Streptococcus sp. co-infection was related to lung lesions of varying degrees and lower slaughter and carcass weight.

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.

The challenge of detecting herds sub-clinically infected with Actinobacillus pleuropneumoniae

The introduction into a naïve herd of animals sub-clinically infected with Actinobacillus pleuropneumoniae (App) is frequently the cause of clinical pleuropneumonia and the identification of such infected herds is a priority in the control of disease. Different serological tests for App have been developed and a number of these are routinely used. Some are species-specific whereas others identify more specifically the serotype/serogroup involved which requires updated information about important serotypes recovered from diseased pigs in a given area/country. Serotyping methods based on molecular techniques have been developed lately and are ready to be used by most diagnostic laboratories. When non-conclusive serological results are obtained, direct detection of App from tonsils is sometimes attempted. This review addresses different techniques and approaches used to monitor herds sub-clinically infected by this important pathogen.

Isolation and genetic characterization of an Actinobacillus pleuropneumoniae serovar K12:O3 strain

An atypical Actinobacillus pleuropneumoniae serovar 12 strain, termed QAS106, was isolated from a clinical case of porcine pleuropneumonia in Japan. An immunodiffusion (ID) test identified the strain as serovar 12. However, the ID test also demonstrated that strain QAS106 shared antigenic determinants with both the serovar 3 and 15 reference strains. Strain QAS106 was positive in the capsular serovar 12-specific polymerase chain reaction (PCR) assay, while the PCR toxin gene profiling and omlA PCR typing assays indicated that strain QAS106 was similar to serovar 3. The nucleotide sequence of the 16S ribosomal DNA (rDNA) of strain QAS106 was identical with that of serovars 3 and 12, but it showed 99.7% identity with that of serovar 15. Nucleotide sequence analysis revealed that genes involved in biosynthesis of the capsular polysaccharide (CPS) of strain QAS106 were identical to those of serovar 12 at the amino acid level. On the other hand, strain QAS106 would express putative proteins involved in the biosynthesis of lipopolysaccharide (LPS) O-polysaccharide (O-PS), the amino acid sequences of which were identical or nearly identical to those of serovars 3 and 15. In conclusion, strain QAS106 should be recognized as K12:O3, even though typical serovar 12 strains are K12:O12. The emergence of an atypical A. pleuropneumoniae serovar 12 strain expressing a rare combination of CPS and O-PS antigens would hamper precise serodiagnosis by the use of either CPS- or LPS-based serodiagnostic methodology alone.

An indirect enzyme-linked immunosorbent assay for detection of antibodies to Actinobacillus pleuropneumoniae serovar 7 in pig serum

Lipopolysaccharide (LPS) antigen was purified from Actinobacillus pleuropneumoniae serovar 7 by phenol-water extraction and fractionated on a, S-100 Sephacryl column. High molecular weight fractions of LPS purified from the S-100 column were pooled and used as antigen in an indirect serovar 7 ELISA. The ELISA was evaluated with sera from pigs experimentally infected with 11 different A. pleuropneumoniae serovars of biotype 1. Estimation of sensitivity and specificity of the A. pleuropneumoniae serovar 7 ELISA was performed using pig sera from herds naturally infected with A. pleuropneumoniae serovar 7 as well as sera from herds free of infection with A. pleuropneumoniae serovar 7. When compared to the complement fixation test (CFT) as a reference test, the ELISA showed much higher sensitivity and statistically equivalent specificity.

Development and evaluation of a mixed long-chain lipopolysaccharide based ELISA for serological surveillance of infection with Actinobacillus pleuropneumoniae serotypes 2, 6 and 12 in pig herds

The objective was to develop an enzyme-linked immunosorbent assay (ELISA) for simultaneous detection of antibodies against Actinobacillus pleuropneumoniae (Ap) serotypes 2, 6 and 12. The assay was designated MIX-ELISA. Lipopolysaccharide (LPS) from Ap serotypes 2, 6 and 12 was purified using hot phenol-water extraction followed by fractionation by size-exclusion chromatography. A mixture of fractions containing molecules with molecular weight above 50 kDa from all three serotypes was used as antigen. The MIX-ELISA was evaluated with sera from pigs experimentally infected with the serotypes 1, 2, 5b, 6, 7, 8, 10 and 12 of Ap biotype 1. In addition to reaction with sera from pigs inoculated with Ap serotypes 2, 6 and 12, reaction was observed with sera from pigs inoculated with serotype 8. Furthermore, the sensitivity and specificity of the test on a herd level were evaluated with sera from herds naturally infected with serotypes 2, 6 or 12 and with sera from herds free of infection with any Ap serotype of biotype 1. The ELISA showed a high herd sensitivity (0.98; 95% confidence interval: 0.89-1.00) and specificity (0.95; 0.88-0.99). The high diagnostic sensitivity and specificity of the assay indicate that screening of herds for Ap infection can be performed using this ELISA. Efficient serological surveillance can be achieved by using such mixed antigen ELISAs coated with size-selected LPS-antigens from the most prevalent serotypes.

Detection of antibodies to Actinobacillus pleuropneumoniae serotype 12 in pig serum using a blocking enzyme-linked immunosorbent assay

The objective was to develop a blocking enzyme-linked immunosorbent assay (ELISA) for detection of antibodies to Actinobacillus pleuropneumoniae (Ap) serotype 12 in pig serum. Lipopolysaccharide (LPS) from Ap serotype 12 was purified and used as antigen in the assay. Antibodies to the LPS antigen in samples of pig serum were detected by inhibition of the binding of polyclonal rabbit antibodies raised against Ap serotype 12. The assay was evaluated against sera from experimentally infected pigs, from pig herds naturally infected with Ap and from herds declared free of Ap serotype 12 infection. The blocking ELISA showed no cross-reaction when tested with sera from pigs experimentally infected with 12 other serotypes of Ap biotype 1. The sensitivity and specificity of the blocking ELISA on the herd level was evaluated by testing sera from pig herds naturally infected with Ap serotypes 2 and/or 12 and from herds declared free of infection with Ap. The Ap serotype 12 blocking ELISA showed a herd sensitivity of 0.77 (95% confidence interval, 0.62-0.88) and a herd specificity of 1.00 (0.95-1.00) with a cut-off value at 40% relative absorbance or 60% inhibition. The assay may be used advantageously as a confirmatory test in serological surveillance programmes for Ap infections in SPF systems for pig production.

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.

Avaliação de testes de ELISA para o diagnóstico sorológico de infecções pelos sorotipos 3, 5 e 7 de Actinobacillus pleuropneumoniae em suínos

Compararam-se dois antígenos no teste de ELISA para o diagnóstico sorológico dos sorotipos 3, 5 e 7 de A. pleuropneumoniae prevalentes no Brasil. Um compunha-se da fase aquosa da extração fenólica de suspensão bacteriana (FAF) e o outro de lipopolissacarídeos de cadeia longa (LPS-CL). Com esses antígenos foram padronizados ELISAs monovalente e polivalente para os sorotipos prevalentes no Brasil. Com os resultados dos testes de um conjunto de amostras de soro de suínos livres de infecção para A. pleuropneumoniae e um conjunto de amostras de soro obtidas de leitões inoculados com os sorotipos citados e que apresentaram soroconversão, determinaram-se as equações discriminantes para os conjuntos de soros e compararam-se os testes quanto à distância generalizada de Mahalanobis, ao coeficiente de determinação, ao teste F, ao coeficiente global, à sensibilidade e à especificidade. Na análise desses parâmetros observou-se que o antígeno FAF foi superior. Com o ELISA PFAF reações positivas não esperadas foram observadas com animais inoculados com os sorotipos heterólogos 2 e 9, não observadas com o PLPS-CL. A sensibilidade dos testes polivalentes ficou entre 91,5 e 95,7% com especificidade similar, indicando que ambos os testes são adequados para triagem sorológica uma vez que detectam os sorotipos 3, 4, 5, 6, 7 e 8.

Serologic detection of Actinobacillus pleuropneumoniae in swine by capsular polysaccharide-biotin-streptavidin enzyme-linked immunosorbent assay

Serologic detection of Actinobacillus pleuropneumoniae infections in swine have been problematic due to antigenic cross-reactivity of Apx toxins, lipopolysaccharide, and outer membrane proteins between A. pleuropneumoniae serotypes and other bacterial species. To maximize serologic specificity and sensitivity, we developed an assay that uses highly purified A. pleuropneumoniae capsular polysaccharide (CP) conjugated to biotin, which is then bound to streptavidin-coated enzyme-linked immunosorbent assay (CP-BS-ELISA) plates. This assay was used to test a panel of 240 serum samples from pigs prior to challenge, after challenge with bacterial species other than A. pleuropneumoniae, or after challenge with A. pleuropneumoniae serotype 1, 5, or 7. Overall assay results for the individual sera tested were reproducible on the same day and on separate days. The sensitivity of the assay was 100% by ELISAs with biotin-CPs of serotypes 1 and 7 and 87.5% by ELISAs with biotin-CP of serotype 5. Specificity was 100% by ELISAs with biotin-CPs of serotypes 1 and 5 and 94.5% by ELISAs with biotin-CP of serotype 7. The biotin-CPs of at least three A. pleuropneumoniae serotypes could be combined for use in a screening assay to detect antibodies to CPs from strains of different serotypes. In conclusion, the CP-BS-ELISA proved to be a serotype-specific and species-specific assay with high sensitivity for the identification of pigs exposed to A. pleuropneumoniae.

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.

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.

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.

Swine model of Haemophilus ducreyi infection

Haemophilus ducreyi is a strict human pathogen that causes sexually transmitted genital ulcer disease. We infected domestic swine with H. ducreyi 35000, resulting in the development of cutaneous ulcers histologically resembling human chancroid lesions. Intraepidermal lesions progressed from pustules to ulcers containing polymorphonuclear leukocytes and were accompanied by a dermal inflammatory infiltrate containing T cells and macrophages. H. ducreyi was recovered from lesions up to 17 days after inoculation, and pigs did not develop immunity to reinfection with the challenge strain. Features of the model include inoculation through abrasions in the epidermis, ambient housing temperatures for infected pigs, the ability to deliver multiple different inocula to a single host, and the availability of monoclonal antibodies against porcine immune cells permitting immunohistochemical characterization of the host immune response to H. ducreyi infection.

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.