A rapid micro-method for the study of antibody-mediated killing of bacteria, with specific application to infection of sheep with Pasteurella haemolytica

Association of LPS chemotype of Mannheimia (Pasteurella) haemolytica A1 with disease virulence in a model of ovine pneumonic pasteurellosis

Host responses during pneumonic pasteurellosis were compared in sheep infected with strains of Mannheimia (Pasteurella) haemolytica A1 differing in their O-antigen type. Nine-week-old, specific pathogen-free lambs were infected intratracheally with parainfluenza type 3 virus (108 TCID50) followed 7 days later by 5—6 . 107 CFU of M. haemolytica A1 possessing rough (group R, 6 lambs) or smooth (group S, 6 lambs) lipopolysaccharide, or saline (group C, 4 lambs). Group C lambs remained afebrile with no evidence of endotoxaemia or bacteraemia and biochemical parameters were normal. Group R and group S lambs became febrile within 2—3 h postinfection and the response was higher and more prolonged in group R lambs. Four group R and 2 group S lambs developed clinical pneumonic pasteurellosis within 24—48 h and the severity of disease correlated with episodes of endotoxaemia, bacteraemia and elevated eicosanoid concentrations. At postmortem, M. haemolytica (107—109 CFU/g) was isolated from the lungs of all 6 group R lambs but from only 1 group S lamb. The results indicate an association between the incidence and severity of ovine pneumonic pasteurellosis and LPS chemotype and suggest an important role for LPS chemotype in determining host-species susceptibility to lung infection.

Production of a d-glycero-d-manno-heptosyltransferase mutant of Mannheimia haemolytica displaying a veterinary pathogen specific conserved LPS structure; development and functionality of antibodies to this LPS structure

Previous structural studies of the lipopolysaccharides from the veterinary pathogens Mannheimia haemolytica (Mh), Actinobacillus pleuropneumoniae (Ap) and Pasteurella multocida (Pm) had identified a conserved inner core oligosaccharide structure that was present in all strains investigated. In order to examine the potential of this inner core structure as a vaccine, a mutagenesis strategy was adopted to interrupt a D-glycero-D-manno-heptosyltransferase gene (losB) of Mh. This gene encodes the enzyme responsible for the addition of a D-glycero-D-manno-heptose residue, the first residue beyond the conserved inner core, and its inactivation exposed the conserved inner core structure as a terminal unit on the mutant LPS molecule. Subsequent analyses confirmed the targeted structure of the mutant LPS had been obtained, and complementation with losB in trans confirmed that the losB gene encodes an alpha-1,6-D-glycero-D-manno-heptosyltransferase. Monoclonal antibodies raised in mice to this LPS structure were found to recognise LPS and whole-cells of the truncated mutant and wild-type Mh. The antibodies were bactericidal against a wild-type Mh strain and were able to passively protect mice in a model of Mh disease. This illustrates that it is possible to raise functional antibodies against the conserved inner core LPS structure.

Characterisation and biological activity of monoclonal antibodies specific for Pasteurella haemolytica A1 capsule and lipopolysaccharide

Monoclonal antibodies (mAb) against both Pasteurella haemolytica A1 capsule and lipopolysaccharide (LPS) were produced. Anti-capsule mAb reacted with the homologous A1 serotype only, whereas mAb against LPS reacted with P. haemolytica serotypes A2, A5, A8, A12, A14 and A16 but not with 33 bacterial species or rough LPS mutant strains tested. Both capsule and LPS antigens were visualised on the surface of bacteria by immunogold electron microscopy. Neither of the mAbs demonstrated antibody-dependent complement-mediated killing in vitro but both facilitated phagocytosis in vitro.

The susceptibility of in vivo-grown Pasteurella haemolytica to ovine defence mechanisms in vitro

Pasteurella haemolytica organisms grown in vivo were examined for their susceptibility to ovine immune mechanisms in vitro. Compared with in vitro grown organisms they were less susceptible to opsonophagocytosis and, in contrast, susceptible to complement-dependent killing in the absence of exogenous antibody. These differences were not associated with phenotypic changes in the surface of the bacterial cell. However, overproduction and de novo synthesis of proteins was observed in in vivo grown organisms. Also, bound host-immunoglobulin was observed on in vivo grown organisms and a role for this in modifying the interaction with immune mechanisms is discussed.

Bactericidal activity in the sera of mice vaccinated with Pasteurella multocida type A

The susceptibility of Pasteurella multocida to killing by serum and the ability of protective vaccines to stimulate this mechanism of immunity in mice were investigated. P. multocida type of bovine origin was used to prepare a vaccine incorporating heat killed organisms and for homologous infection of mice. Bactericidal capacity and ELISA antibody titres were determined for individual mouse sera. Protection was clearly associated with bactericidal antibodies raised by vaccination. The bactericidal assay may be useful as a rapid, simple screening test of vaccinated mice for functional protective antibody levels.

Protection of lambs against experimental pneumonic pasteurellosis by transfer of immune serum

Passive protection of specific pathogen-free lambs against experimental pasteurellosis was achieved using antisera from conventionally reared sheep which were either convalescent from experimental pneumonia or inoculated with Pasteurella haemolytica A2 vaccines. The complete immune sera, or immunoglobulin-rich fractions prepared from them, when administered separately or together provided 94-100% protection of recipients compared to control lambs. Antibodies to P. haemolytica in donor sera were quantified by anti-sodium salicylate extract (SSE) and anti-lipopolysaccharide (LPS) ELISA, bactericidal assay, cytotoxin neutralization and indirect haemagglutination. The anti-SSE ELISA titres correlated best with protective efficacy and could be used to measure antibody in recipient lambs immediately before challenge. The degree of protection was unaffected by prior infection with parainfluenza virus Type 3, suggesting that such exposure did not enhance exudation of circulating immunoglobulin into the respiratory tract. It was concluded that systemic humoral immunity alone can prevent pasteurellosis.

A crude cytotoxin vaccine protects sheep against experimental Pasteurella haemolytica serotype A2 infection

Three vaccines containing Pasteurella haemolytica serotype A2 antigens were tested for their ability to protect sheep against a homologous challenge. A crude cytotoxin preparation in combination with a sodium salicylate extract (SSE) or crude cytotoxin alone were highly protective (98 and 86%, respectively), whereas SSE alone was poorly (47%) protective. These findings indicated that the crude cytotoxin was an essential component of a protective vaccine. Protection correlated with serum cytotoxin-neutralising (CN) titres and bactericidal activity, which were stimulated by antigens in the crude cytotoxin preparation.