Mechanisms involved in protection provided by immunization against core lipopolysaccharides of Escherichia coli J5 from lethal Haemophilus pleuropneumoniae infections in swine

Serum antibody responses in horses and mice following immunization with Actinobacillus equuli outer membrane proteins and recombinant Aqx toxin

The immune responsiveness of mice (without prior natural exposure) and mares (with naturally acquired antibodies) was determined following vaccination with Actinobacillus equuli outer membrane proteins (OMPs) and/or recombinant A. equuli toxin (rAqx). Mice were vaccinated subcutaneously on days 0 and 21 with one of three doses (5, 25 or 50μg) of A. equuli OMPs, rAqx or both, together with Freund's incomplete adjuvant (FIA). Antibodies against formalin-killed whole bacterial cells (WBCs), OMPs and Aqx were determined on days 0, 21 and 42. Mares were vaccinated subcutaneously on days 0 and 21 with 100μg OMPs, 100μg rAqx or a combination of 50μg of each antigen, together with FIA. Antibodies against WBCs, OMPs and Aqx were determined at 7day intervals for the first 42days, as well as on days 56, 70, 154 and 238. Vaccination of mice stimulated an apparent dose response to OMPs and Aqx. Antibodies against OMPs and Aqx were enhanced following vaccination of mares that had naturally acquired pre-existing antibodies. There was no evidence of interference with antibody responses to the individual antigens when OMPs and rAqx were combined prior to vaccination.

Evaluation of multicomponent recombinant vaccines against Actinobacillus pleuropneumoniae in mice

Background

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

Methods

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

Results

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

Conclusion

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

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.

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.

Susceptibility to Actinobacillus pleuropneumoniae infection in pigs from an endemically infected herd is related to the presence of toxin-neutralizing antibodies

Our objective was to identify pigs of an endemically infected herd that were susceptible to pleuropneumonia due to Actinobacillus pleuropneumoniae. The presence of toxin-neutralizing antibodies was studied in serum of 36 pigs from birth until 24 weeks of age. Titers gradually declined during the first twelve weeks of life and increased thereafter. Sera from one-hundred 3-weeks-old piglets and one-hundred 20-weeks-old pigs were sampled and neutralization titers were determined. From each group we selected 5 pigs with the lowest titers and 5 pigs with the highest titers. These selected pigs (n = 20) were inoculated endobronchially with A. pleuropneumoniae. Pigs that survived from infection were necropsied after 48 h. Pigs with low neutralization titers had severe lung lesions, whereas pigs with high titers had no or minor lung lesion. These differences were significant (P < 0.05). From this field study we conclude that susceptibility to Actinobacillus pleuropneumoniae can be predicted by absence of toxin-neutralizing antibodies.

Antigenic crossreactivity and lipopolysaccharide neutralization properties of bovine immunoglobulin G

We investigated a possible mechanism by which immunization against core and lipid A determinants of lipopolysaccharide reduced clinical cases of mastitis and symptoms commonly associated with heterologous Gram-negative IMI. The IgG fraction of sera from cows immunized with either Escherichia coli J5 bacterin, E. coli J5 lipopolysaccharide conjugate vaccine, or unimmunized controls was purified by precipitation with caprylic acid and ammonium sulfate. The degree of IgG crossreactivity with Gram-negative bacteria that were isolated from clinical quarters was greater than that with Gram-positive isolates of Staphylococcus aureus. The highest magnitude of crossreactivity was against smooth strain E. coli isolates, followed by heterologous species of Enterobacter, Serratia, and Klebsiella isolates. Serum IgG from cows immunized with conjugate was highly crossreactive to E. coli J5, E. coli O111:B4, Serratia marcescens, Klebsiella pneumoniae, and Salmonella typhimurium lipopolysaccharides. The magnitude of antibody crossreactivity with lipopolysaccharides coincided with the ability of IgG to suppress the mitogenic effect of lipopolysaccharides on bovine lymphocytes.

Tissue reaction and immunity in swine immunized with Actinobacillus pleuropneumoniae vaccines

These studies were done to develop a subunit vaccine for swine that would protect against disease, but not create unacceptable tissue reactions at the immunization site. Swine were used to evaluate the local effects of subunit vaccines prepared from extracts of Actinobacillus pleuropneumoniae serotype 1 containing one of a wide variety of adjuvants. The antigen was an anionic fraction of a saline extract of A. pleuropneumoniae (ANEX). The adjuvants used were vegetable oils (peanut, sesame, canola, or corn oils, vitamin E, or Lipposyn II emulsion); mineral oil (Marcol-52) and other materials (aluminum hydroxide, polyethylene glycol, Quil-A, Amphigen, or Emulsigen-Plus). Two types of experiments were done. In the 1st set of experiments, pigs were given multiple simultaneous injections in different sites and euthanized on days 1, 3, 7, 14, 21, or 28. Tissues were examined for gross and histopathological lesions. In the 2nd set of experiments, 48 pigs were allocated to 6 groups and vaccinated twice with a vaccine containing ANEX antigen combined with one of various adjuvants. Antibody responses and protection from challenge were evaluated. Among the adjuvants that were tested, mineral oils induced protective immunity, although the mineral oil Marcol-52 resulted in severe tissue reactions. The vegetable oils induced little protective immunity, and some of them were quite irritating. The response to the other materials ranged from little irritation or protection induced by the vaccine containing aluminum hydroxide to effective protection without irritation after vaccination with ANEX/Amphigen or ANEX/Emulsigen-Plus combinations. In conclusion, swine were protected against disease by a subunit vaccine that did not create unacceptable tissue reaction at the immunization site.

Convalescent pigs are protected completely against infection with a homologous Actinobacillus pleuropneumoniae strain but incompletely against a heterologous-serotype strain

To study whether Actinobacillus pleuropneumoniae induces a species-specific immunity, we infected pigs in the left lung with serotype 3 or 9 and after 3 weeks we infected their right lungs with serotype 9. Convalescent pigs were protected against homologous strain reinfection, but after heterologous strain reinfection the degree of protection varied. Neutralizing antibodies were not essential for protection.

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.

Protection of mice and swine against infection with Actinobacillus pleuropneumoniae by vaccination

CaCl2 and LiCl cell extracts and a crude hemolysin preparation were isolated from Actinobacillus pleuropneumoniae serotype 1 strain 4074 and tested for protection against A. pleuropneumoniae serotype 1 and 5 in mice. The LiCl cell extract adsorbed on AlPO4 and the crude hemolysin preparation adsorbed on Al(OH)3 showed a highly significant protection (P < 0.01) against both serotypes. Different vaccine preparations were used to immunize pigs by intra-muscular injection at days 0 and 14; the pigs were then challenged at day 21 by intra-tracheal inoculation of 1 x 10(8) colony forming units (CFU) of a serotype 1 strain 4074. A vaccine which combined the LiCl extract and the crude hemolysin preparation adsorbed on Al(OH)3 gave the best protection with no mortality and no sign of morbidity in the vaccinated pigs. In the other experimental groups which included a group immunized with a commercial bacterin, mortality, respiratory disease and extensive pulmonary lesions were noted. This mixture shows good potential as a vaccine against pleuropneumonia in pigs.

Antigenic homology of endotoxin with a coliform mastitis vaccine strain, Escherichia coli O111:B4 (J5)

This study examined recognition of heterologous Gram-negative endotoxin by antibodies recognizing common lipopolysaccharide core antigens. Gram-negative endotoxins from 11 heterologous bacterial strains were tested for recognition by antibodies against common lipopolysaccharide core antigens. Serum was harvested from a calf immunized with the Rc mutant, Escherichia coli O111:B4 (J5), and affinity purified against endotoxin derived from an Ra mutant, Salmonella typhimurium, producing an antibody reagent recognizing homologous Gram-negative core antigens present in the Rc mutant vaccinal antigen. This reagent demonstrated reactivity against 11 chemically purified Gram-negative endotoxins. Included were endotoxins derived from 3 smooth E. coli species, 2 Salmonella spp., Shigella flexneri, Klebsiella pneumoniae, Pseudomonas aeruginosa, Serratia marcescens, and lipid A. Endotoxin derived from K. pneumoniae had significantly higher ELISA reactivity with core antigen specific antibodies than did endotoxin derived from either E. coli O111:B4 (J5) or P. aeruginosa. These results suggest immunization with R mutant bacterins may have utility in the prevention of Gram-negative mastitis even when whole bacteria react poorly with antibodies recognizing common core antigens.

Structural relatedness of enteric bacterial porins assessed with monoclonal antibodies to Salmonella typhimurium OmpD and OmpC

The immunochemistry and structure of enteric bacterial porins are critical to the understanding of the immune response to bacterial infection. We raised 41 monoclonal antibodies (MAbs) to Salmonella typhimurium OmpD and OmpC porin trimers and monomers. Enzyme-linked immunosorbent assays, immunoprecipitations, and/or Western immunoblot techniques indicated that 39 MAbs (11 anti-trimer and 28 anti-monomer) in the panel are porin specific and one binds to the lipopolysaccharide; the specificity of the remaining MAb probably lies in the porin-lipopolysaccharide complex. Among the porin-specific MAbs, 10 bound cell-surface-exposed epitopes, one reacted with a periplasmic epitope, and the remaining 28 recognized determinants that are buried within the outer membrane bilayer. Many of the MAbs reacting with surface-exposed epitopes were highly specific, recognizing only the homologous porin trimers; this suggests that the cell-surface-exposed regions of porins tends to be quite different among S. typhimurium OmpF, OmpC, and OmpD porins. Immunological cross-reaction showed that S. typhimurium OmpD was very closely related to Escherichia coli NmpC and to the Lc porin of bacteriophage PA-2. Immunologically, E. coli OmpG and protein K also appear to belong to the family of closely related porins including E. coli OmpF, OmpC, PhoE, and NmpC and S. typhimurium OmpF, OmpC, and OmpD. It appears, however, that S. typhimurium "PhoE" is not closely related to this group. Finally, about one-third of the MAbs that presumably recognize buried epitopes reacted with porin domains that are widely conserved in 13 species of the family Enterobacteriaceae, but apparently not in the seven nonenterobacterial species tested. These data are evaluated in relation to host immune response to infection by gram-negative bacteria.

Protective local and systemic antibody responses of swine exposed to an aerosol of Actinobacillus pleuropneumoniae serotype 1

The isotype-specific antibody responses in serum and in nasal and pulmonary lavage fluids of swine following aerosol immunization with an attenuated strain of Actinobacillus pleuropneumoniae serotype 1, strain CM5A, was investigated. The presence of immunoglobulin G (IgG), IgA, and IgM with specificities for capsular polysaccharide, lipopolysaccharide, and hemolysin was determined by enzyme-linked immunosorbent assay by using purified antigens. Strain CM5A induced serum antibodies of each isotype to the three antigens. The serum antibody response was sustained and typical of persistent antigenic stimulation. The specific IgM response decreased and the specific IgG response increased after challenge with strain CM5. IgA specific for the three antigens was detected in nasal secretions from all immune pigs, whereas specific IgG could only be detected in samples contaminated with blood. Both IgA and IgG specific for each of the antigens were detected in pulmonary lavage samples. There was no significant increase in specific IgA in nasal secretions; however, levels of lipopolysaccharide-specific and hemolysin-specific IgG and IgA in pulmonary secretions rose after aerosol challenge with strain CM5. Passive transfer of immune swine serum resulted in protection against pleuropneumonia and in levels of specific serum IgG which were similar to those in actively immunized pigs. It is concluded that specific serum IgG antibodies are important in protection from porcine pleuropneumonia.

Optimization and standardization of an enzyme-linked immunosorbent assay protocol for serodiagnosis of Actinobacillus pleuropneumoniae serotype 5

An indirect enzyme-linked immunosorbent assay protocol has been optimized with special emphasis given to assay standardization and quality control. Technical aspects such as choice of a microplate, antigen immobilization, buffer composition, optimal screening dilution of sera, and kinetics of the enzymatic reaction were studied and evaluated in order to design a standard protocol offering maximal analytical sensitivity and specificity, as well as to obtain minimal within- and between-plate variability. Among the 27 plates tested, the Nunc 475-094 and 269-620 immunoplates were found to be the best in terms of high positive-to-negative ratio and low variability. No significant differences in antigen immobilization were found by using buffers of various compositions or pHs; however, the presence of magnesium ions (Mg2+; 0.02 M) resulted in a twofold increase in nonspecific background. An optimal screening dilution of sera was established at 1:200. A 1-h incubation period for test serum was found to be optimal. Maximum enzymatic activity for peroxidase was obtained by adjusting both substrate (H2O2) and hydrogen donor [2,2' -azinobis(3-ethylbenz-thiazoline sulfonic acid)] concentrations to 4 and 1 mM, respectively. To control between-plate variability, a timing protocol was adopted. Within-plate variability was also controlled by using a sample placement configuration pattern. Sliding scales were determined by repeated testing of a cross section of samples to set acceptance limits for both within- and between-plate variability. These limits were used in a quality control program to monitor assay performance. The results obtained suggest that this standardized protocol might be useful in the serodiagnosis of Actinobacillus pleuropneumoniae serotype 5.

Evolution of the ferric enterobactin receptor in gram-negative bacteria

Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis of iron-deficient and replete cell envelopes, 59Fe-siderophore uptake studies, and Western immunoblots and cytofluorimetric analyses with monoclonal antibodies (MAbs), we surveyed a panel of gram-negative bacteria to identify outer membrane proteins that are structurally related to the Escherichia coli K-12 ferric enterobactin receptor, FepA. Antibodies within the panel identified FepA epitopes that are conserved among the majority of the bacteria tested, as well as epitopes present in only a few of the strains. In general, epitopes of FepA that are buried in the outer membrane bilayer were more conserved among gram-negative bacteria than epitopes that are exposed on the bacterial cell surface. The surface topology and tertiary structure of FepA are quite similar in E. coli and Shigella flexneri but differ in Salmonella typhimurium. Of the 18 different genera tested, 94% of the bacteria transported ferric enterobactin, including members of the previously unrecognized genera Citrobacter, Edwardsiella, Enterobacter, Haemophilus, Hafnia, Morganella, Neisseria, Proteus, Providencia, Serratia, and Yersinia. The ferric enterobactin receptor contains at least one buried epitope, recognized by MAb 2 (C. K. Murphy, V. I. Kalve, and P. E. Klebba, J. Bacteriol. 172:2736-2746, 1990), that is conserved within the structure of an iron-regulated cell envelope protein in all the bacteria that we have surveyed. With MAb 2, we identified and determined the Mr of cell envelope antigens that are immunologically related to E. coli FepA in all the gram-negative bacteria tested. Collectively, the library of anti-FepA MAbs showed unique patterns of reactivity with the different bacteria, allowing identification and discrimination of species within the following gram-negative genera: Aeromonas, Citrobacter, Edwardsiella, Enterobacter, Escherichia, Haemophilus, Hafnia, Klebsiella, Morganella, Neisseria, Proteus, Providencia, Pseudomonas, Salmonella, Serratia, Shigella, Vibrio, and Yersinia.

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.

Antigenic homology among gram-negative organisms isolated from cattle with clinical mastitis

This study examined the degree of serologic homology among mastitis pathogens. Antibodies were raised against the Rc mutant, Escherichia coli O111:B4 (strain J5) and affinity purified against lipopolysaccharide derived from the Ra mutant, Salmonella typhimurium TV119. These antibodies reacted with a battery of unrelated Gram-negative bacteria in whole cell ELISA. Bacteria with strong cross-reactions included a heterologous, smooth E. coli, Salmonella dublin, S. typhimurium, Salmonella newport, and Pseudomonas aeruginosa. Recognition of Klebsiella pneumoniae and Bordetella bronchisepticum was observed, but reactions were weaker than with the other isolates. The reduced recognition of these isolates probably reflects a masking effect of the bacterial capsule and variations in lipopolysaccharide structure. The polyclonal antibody did not recognize a Gram-positive isolate, Staphylococcus aureus. These immunoglobulins were then tested using whole cell ELISA against a panel of bacteria recovered from the mammary glands of cattle with clinical mastitis. Marked reactivity was noted against a variety of Gram-negative pathogens. Gram-positive isolates had lower recognition by Gram-negative core antigen specific immunoglobulin. The results suggest immunization with rough mutant bacteria may have broad application in the prevention of coliform mastitis.

Bacteraemia in man and animals: an overview

Bacteraemia signifies invasion of the bloodstream by bacteria. In most systemic infections in man and animals, bacteria enter the blood at some stage during the infection and are rapidly distributed throughout the body. A wide variety of organisms have been associated with bacteraemia. Prompt detection of the aetiological agents of bacteraemia is of prime importance in clinical microbiology. There are no defined recommendations for blood cultures in animals but both conventional and improved methods are available for detection of bacteraemia in man. The consequences of bacteraemia are as diverse as the potential aetiological agents. Monoclonal antibodies to the core glycolipid (lipid A) seems very promising for the treatment of bacteraemia and septic shock caused by Gram-negative bacteria in man and animals.

Humoral antibody response and protective immunity in swine following immunization with the 104-kilodalton hemolysin of Actinobacillus pleuropneumoniae

Five cesarean-derived, colostrum-deprived pigs were given three adjuvant-supplemented subcutaneous and one intravenous injection of the purified 104-kDa hemolysin from serotype 1 Actinobacillus pleuropneumoniae CM-5. Six control animals received phosphate-buffered saline only. Five of six control pigs died within 24 h after challenge. The sixth control pig was moribund and euthanized after 48 h. All six pigs had pleuropneumonia, and A. pleuropneumoniae was isolated from all six lungs. None of the vaccinated pigs died as a result of challenge. After being euthanized, two pigs in this group had no lung lesions but three had chronic pleuropneumonia involving 10, 20, and 40% of the lung tissue. A. pleuropneumoniae was isolated from lung lesions of these three animals but not from the two pigs without lesions. The prechallenge hemolysin-neutralizing antibody titers in the vaccinated pigs were 1:10,900, 1:10,600, 1:4,800, 1:3,900, and 1:3,000, in order of increasing lung involvement. None of the control pigs had neutralizing antibodies. Enzyme-linked immunosorbent assay (ELISA) antibodies to capsule, lipopolysaccharide, and hemolysin were not detected in serum samples collected from the control pigs. In the vaccinated group, prechallenge sera did not contain ELISA antibodies to capsule or lipopolysaccharide. ELISA antibodies to the hemolysin were detected only in the prechallenge and postchallenge serum samples. These results indicate that pigs immunized with the 104-kDa hemolysin of serotype 1 A. pleuropneumoniae are protected against challenge with virulent bacteria. The association between neutralizing antibodies and protection indicates indirectly that the hemolysin is an important virulence factor.

Actinobacillus pleuropneumoniae hlyX gene homology with the fnr gene of Escherichia coli

The hlyX gene from Actinobacillus pleuropneumoniae, which confers a hemolytic phenotype on Escherichia coli, was sequenced, and its role in regulation of gene expression was investigated. No similarity was found between the hlyX sequence and sequences of known hemolysin or cytotoxin genes. However, the hlyX sequence was very similar to that of the fnr gene of Escherichia coli which encodes the global regulatory protein, FNR. Comparison of the deduced amino acid sequence of the hlyX gene product (HlyX) with that of FNR revealed a high degree of well-aligned sequence correlation throughout the polypeptide chain. For example, 23 of 24 amino acids in the DNA-binding region of FNR are identical in the corresponding region of HlyX. Four cysteine residues in the amino-terminal region are also conserved. The promoter region of hlyX is very similar to that of fnr. It has a putative -10 sequence which closely resembles the E. coli -10 consensus sequence. This sequence is overlapped by a potential operator which is very similar to the FNR-binding-site consensus sequence. Functional homology between HlyX and FNR was also demonstrated. Plasmids carrying hlyX complemented the nutritional lesion of an fnr deletion strain of E. coli. These data suggest that HlyX may regulate, rather than mediate, hemolytic activity in E. coli, but the possibility that HlyX is both a regulator of gene expression and a hemolysin cannot be excluded.

Efficacy of a cell extract from Actinobacillus (Haemophilus) pleuropneumoniae serotype 1 against disease in swine

We partially characterized a cell extract (CE) from Actinobacillus pleuropneumoniae serotype 1 and used the CE to test the efficacy of secreted proteins against disease. Secreted products from 4-h culture supernatants were precipitated with 20% polyethylene glycol. Analysis of the CE indicated the presence of protein, endotoxin, and carbohydrate. Hemolytic activity to bovine erythrocytes and cytotoxic activity to porcine mononuclear leukocytes was also demonstrated. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the CE from a 4-h culture showed a major band at 110 kilodaltons (kDa), while a CE of a 26-h culture indicated the presence of a number of additional proteins, including the 110-kDa protein. The 110-kDa protein was also identified as a glycoprotein by periodic acid-Schiff and silver staining. A single band precipitated against convalescent-phase pig antiserum when the polyethylene glycol precipitate was used in an Ouchterlony plate. Vaccination with CE conferred greater protection against challenge with the homologous serotype than either a commercial bacterin or an outer membrane protein vaccine. Hemolysin-neutralizing titers were higher both pre- and postchallenge in the group vaccinated with the CE compared with in all other groups. We believe that this demonstrates the importance of secreted factors in protection against disease and suggests that the 110-kDa protein is an important immunogen.

Immunity targeting common core antigens of gram-negative bacteria

Antibodies against common gram-negative core antigens can provide protection against environmental, commensual, and contagious bacteria that cause life-threatening gram-negative sepsis/endotoxemia. Cross-protective immunity may be effective against many common livestock diseases, including neonatal coliform septicemia, coliform mastitis, septic metritis, and severe enteritis. The greatest potential benefit of cross-protective immunity may be realized in the reduction of ill-thrift and mortality of neonates. The use of active and passive immunization against core antigens common to gram-negative bacteria has been successful in human beings, laboratory animals, and livestock. Protection has been observed following challenge with either heterologous endotoxin or unrelated, live, virulent gram-negative bacteria.

Blood gas and hematological changes in experimental peracute porcine pleuropneumonia

The effect of experimental, peracute, porcine pleuropneumonia on arterial blood gases, acid base status, the leukogram, and gross and microscopic lung structure was studied in nine growing pigs (mean weight +/- SD 10.6 +/- 2.0 kg). Pigs were inoculated intranasally with a virulent serotype 5 isolate of Actinobacillus pleuropneumoniae, and all showed signs typical of the disease within four hours. Death occurred in all pigs from 4.5 to 32 hours postinoculation (mean 14 hours). Gross and microscopic changes were typical of porcine pleuropneumonia in all pigs. Changes in the leukogram included a rapid decline in total white cells, segmented neutrophils, lymphocytes, monocytes, and eosinophils. Pigs maintained alveolar ventilation throughout the study as arterial CO2 tension was unchanged; however, arterial O2 tension and pH decreased from (mean +/- SD) 95.2 +/- 5.7 torr and 7.463 +/- 0.018 at baseline to 62.1 +/- 12.3 torr and 7.388 +/- 0.045, respectively, within 90 minutes prior to death. The data showed that in this model of peracute porcine pleuropneumonia, progressive ventilatory failure was not a feature of the disease, and the blood gas values and acid base status were maintained within physiological ranges. The histopathological hematological and physiological findings were consistent with the hypothesis that peracute porcine pleuropneumonia resembles septic shock.

Humoral response in neonatal calves following immunization with Escherichia coli (strain J5): the effects of adjuvant, age and colostral passive interference

Serologic responses in 61 calves 3 to 34 days of age following immunization with bacterins containing a heat-killed rough mutant, Escherichia coli 0111:B4 (strain J5) were determined by an enzyme-linked immunosorbent assay specific for the IgG isotype. Administration of either heat-killed bacteria or oil-based adjuvants alone failed to enhance serologic recognition of common core antigens when comparing to nonvaccinate controls. Increased titers were uniquely and specifically limited to calves receiving the antigen in an oil emulsion. In a second experiment, age and initial, passively acquired titer recognizing the vaccinal antigen were not found to have any effect on the magnitude of the humoral response of 57 calves following immunization.

Molecular cloning and characterization of a hemolysin gene from Actinobacillus (Haemophilus) pleuropneumoniae

This article describes the molecular cloning and expression of a hemolysin gene from a serotype 1 strain of Actinobacillus pleuropneumoniae. The hemolysin was a thermolabile protein with an apparent molecular weight of 29,500 (29.5K hemolysin). Unlike expression of the recently described 105K hemolysin of A. pleuropneumoniae (J. Frey and J. Nicolet, FEMS Microbiol. Lett. 55:41-46, 1988), expression of this hemolysin was not regulated by Ca2+. Antiserum prepared against the 105K hemolysin did not neutralize the activity of the 29.5K hemolysin; conversely, antiserum prepared against the 29.5K hemolysin did not neutralize the activity of the 105K hemolysin. The hemolytic activity was not neutralized with antisera against hemolytic Escherichia coli, Streptococcus agalactiae, or purified streptolysin O, but antisera prepared against recombinants containing the 29.5K gene and convalescent pig sera abrogated hemolytic activity. Although hemolytic activity could be detected in several strains of E. coli K-12 and in minicells expressing several different constructs encoding the 29.5K hemolysin, we could not rigorously exclude the possibility that the gene which we have isolated encodes a regulator of hemolytic activity rather than a hemolysin per se.

Prevention of clinical coliform mastitis in dairy cows by a mutant Escherichia coli vaccine

A prospective cohort study was undertaken in two commercial California dairies. The treatment group, 246 cows, received three doses of a whole cell bacterin of J5 Escherichia coli (mutant of E. coli O111:B4) plus Freund's incomplete adjuvant vaccine (two in the dry period and one after calving) while 240 unvaccinated cows served as controls. Thirty-five cases of clinical coliform mastitis were diagnosed, six in vaccinated cows and 29 in unvaccinated cows. Bacteria isolated from the clinical cases included 15 E. coli five Klebsiella pneumoniae, three K. oxytoca, three K. ozaenae, five Enterobacter aerogenes, three Serratia marcescens and one Serratia spp. Four control cows were culled, three of them because of chronic coliform mastitis and one because of postcoliform infection agalactia. Incidence rate of clinical gram-negative mastitis was 2.57% in vaccinated cows and 12.77% in unvaccinated cows. The estimated risk ratio, the measure of risk of having clinical gram-negative mastitis for vaccinated cows to unvaccinated cows, was 0.20 (p less than 0.005), indicating a strong relationship between vaccination and lack of clinical gram-negative mastitis. The results of this trial indicate that the administration of the E. coli J5 vaccine is protective against natural challenge to gram-negative bacteria, and reduces the incidence of clinical gram-negative mastitis in dairy cows during the first three months of lactation.

Antibody titres to core lipopolysaccharides in horses with gastrointestinal disorders which cause colic

Serum immunoglobulin (Ig) titres to core lipopolysaccharide (LPS) were determined in 102 horses admitted to a university referral hospital during a 12-month period for evaluation of colic. Serum samples were collected again 10-14 days later from 84 of the horses. Titres to core LPS were quantitated by an indirect enzyme-linked immunosorbent assay (ELISA), utilising the J-5 mutant of Escherichia coli 0111:B4 as the solid-phase antigen. All horses had natural antibodies to core LPS at the time of admission and the titre was not affected significantly by age, sex or type of gastrointestinal disorder. The geometric mean titres to core LPS increased significantly within 14 days of admission in those horses with large colon displacement (25), ileal impaction (13), small intestinal strangulating obstruction (11) and small colon obstruction (4). Twenty four (28.6 per cent) of the horses had at least a 4-fold rise in titre (seroconversion) to core LPS within 14 days of admission. The incidence of seroconversion to core LPS was significantly greater (P < 0.05) in horses with disorders requiring surgical intervention (35.8 per cent) than in those with disorders (proximal enteritis, colitis, large colon impaction and unknown) which only required medical treatment (16.1 per cent). Seroconversion rate was not statistically different between groups of horses with diseases of the small intestine which required surgical or medical treatments. The results of this study indicate that gastrointestinal disorders that cause colic in horses result in IgG production to core LPS, and the latter is more prevalent in disorders requiring surgery.

Antibodies to Escherichia coli J5 core glycolipids in gnotobiotic and conventionally reared piglets

Antibody titers to Escherichia coli J5 endotoxin core glycolipids were determined in 5-week-old, unvaccinated, conventionally reared and gnotobiotic pigs in a micro-ELISA. Both groups of pigs had naturally occurring antibody of all major isotypes. The conventionally reared pigs had significantly (P less than 0.05) more serum antibody to J5 endotoxin glycolipids (geometric mean titer, GMT = 2.07) than gnotobiotes (GMT = 0.85). This relationship was not necessarily true in fluids from various anatomical structures. In gnotobiotes, mucosal-associated, anti-J5 glycolipid immunoglobulin was primarily IgM. In conventionally reared piglets, the mucosal-associated, anti-J5 glycolipid antibody contained more IgA. Such naturally occurring antibody could confer partial to complete immunity from challenge with Gram-negative bacteria.

Immune response to vaccination

Effective immune responses requires a synchronization of a number of different physiological and immunological events. Effective vaccines simulate natural invasion of the body by microbes or parasites. The route of administration of vaccines contributes to the effectiveness of preventing or controlling local or systemic infections. Immune responses to microbial agents usually involve more than one mechanism. For instance, toxins may be neutralized by a single antibody, while cellular interactions are required for destruction of more complex microorganisms. These systems have been illustrated in this review.

Development and evaluation of an enzyme-linked immunosorbent assay for endotoxin in milk

A double antibody sandwich enzyme-linked immunosorbent assay (ELISA) was developed for the detection of endotoxin in milk samples. Bovine and rabbit antisera raised in response to vaccination with the J5 mutant of Escherichia coli 0111:B4 were used. Antiserum to this mutant has been shown to be cross-reactive with endotoxin from other gram-negative organisms. Known quantities of endotoxin were added to milk samples to generate a standard curve. Acid treatment of whole milk enhanced the detection of endotoxin as compared to untreated whole milk, skim milk and chloroform-treated milk. Milk samples from experimentally induced mastitic cows were then assayed for endotoxin content. Recovery of endotoxin, as measured by ELISA, positively correlated with the amount of endotoxin infused and the time post-infusion of sampling. However, when endotoxin from these samples was quantitated using the Limulus Amebocyte Lysate (LAL) assay, readings tended to increase, suggesting false-positive reactions with the LAL assay. Milk samples from cases of clinical mastitis were assayed by ELISA with 64% of these showing measurable levels of endotoxin. While further studies of this assay are needed, refinements may produce an assay important for clinical applications.

Virulence properties and protective efficacy of the capsular polymer of Haemophilus (Actinobacillus) pleuropneumoniae serotype 5

The role of the capsule of Haemophilus (Actinobacillus) pleuropneumoniae serotype 5 in bacterial virulence, and the protective efficacy of antibody to serotype 5 capsule was investigated. Encapsulated H. pleuropneumoniae serotype 5 were resistant to killing by complement and antibody to capsule or somatic antigens, whereas a noncapsulated mutant was sensitive to killing by the alternative complement pathway alone. Antiserum to whole H. pleuropneumoniae serotype 5 bacteria or monospecific antiserum to capsule was capable of opsonizing bacteria of the homologous serotype for phagocytosis by swine polymorphonuclear leukocytes but was not opsonic for a heterologous serotype. An immunoglobulin M monoclonal antibody to the serotype 5 capsule was not opsonic for any serotype. Mice were protected against lethal, intranasal challenge with the homologous or heterologous serotype after immunization with live encapsulated or noncapsulated bacteria, but not after immunization with killed bacteria, lipopolysaccharide, or a capsule-protein conjugate vaccine. The protection induced by immunization with live bacteria was transferred to nonimmune, syngeneic mice by serum but not by spleen cells. Nonimmune pigs passively immunized with monospecific swine serum to capsule were protected from lethal infection but not from development of hemorrhagic lung lesions, whereas pigs passively immunized with swine antiserum to live bacteria did not develop severe respiratory lesions. Thus, the capsule of H. pleuropneumoniae serotype 5 was inhibitory to the bactericidal activity of serum and was antiphagocytic. Antibody to the capsule was opsonic but was not fully protective.

Therapy of suspected septicemia in neonatal foals using plasma-containing antibodies to core lipopolysaccharide (LPS)

Equine antiserum to core lipopolysaccharide (LPS) was evaluated in a double-blind prospective study for therapeutic benefit in suspected septicemia in neonatal foals. Forty foals younger than 7 days of age were included in the study by satisfaction of clinical and laboratory criteria, suggestive of gram-negative septicemia. Twenty-two foals were treated with core LPS antiserum (plasma produced from horses which were hyperimmunized with rough gram-negative mutant bacterin) and 18 foals received "nonimmune" plasma (from horses prior to immunization against core LPS). All foals received antimicrobials, fluids, and other supportive care measures, depending on clinical signs and according to accepted current practice. The clinical and laboratory data of each foal were monitored and recorded daily for 14 days after plasma treatment or until death. The overall survival rate of these 40 foals with septicemia was 52.5%. The most prevalent diagnoses in addition to septicemia were enteritis and pneumonia. Of 30 positive bacterial cultures, 93% were due to gram-negative organisms. There was no statistically significant increase in survival rate in the 22 foals given core LPS antiserum (P greater than 0.05).