Search

The ability of mucosally administered antigen to provide protection against Streptococcus equi ('Strangles') infections in horses was examined. First, an enzyme linked immunosorbent assay (ELISA) was developed to detect the immune status of horses to S. equi. This assay was used to select Strangles-naive horses for the study and also to monitor their response to immunisation. Potential vaccine candidates were: (a) orally administered paraformaldehyde killed S. equi; (b) intraperitoneally (IP) administered paraformaldehyde killed S. equi in a non-inflammatory adjuvant; (c) orally administered live avirulent S. equi; (d) orally administered microencapsulated streptococcal M protein. The latter three preparations were first assessed in a rat model, using rate of lung bacterial clearance following intratracheal inoculation of live virulent bacteria as an indication of efficacy. Candidates (a) and (b) were then assessed in an equine model. IP immunisation of horses was shown to effectively induce production of specific antibody in mucosal and systemic sites. Four weeks after initial immunisation, horses were challenged intranasally with live virulent S. equi. Both groups of immunised horses demonstrated partial protection following vaccination. Of the IP immunised horses, only two out of four developed clinical signs of Strangles following live challenge. The orally immunised horses all developed submandibular abscesses containing S. equi. However, none of the immunised horses became as ill as the control horses in terms of fever, anorexia, loss of condition and general malaise.

Rats intestinally immunized with whole killed non-typable Haemophilus influenzae clear this organism from the lungs faster than non-immunized rats. This study investigated the role of antibody and phagocytes in the clearance mechanism. First, dose-response experiments demonstrated that while lowering the dose of non-typable H. influenzae reduced the level of detectable specific antibody in bronchial washings, the ability to accelerate bacterial clearance persisted to much lower doses. Second, specificity experiments showed that intestinal immunization with non-typable H. influenzae cross-protected against Pseudomonas aeruginosa, even though antibodies were not absorbed out of serum by incubation with P. aeruginosa. Third, serum antibody was shown to be bactericidal for non-typable H. influenzae in the presence of complement (P < 0.05), while bronchial washings antibody was not. The bactericidal effect of the serum was abrogated by the addition of bronchial washings. Fourth, an ELISA quenching assay demonstrated that neutrophils from intestinally immunized rats were able to phagocytose more bacteria in a given time period (P < 0.05) than unimmunized rats and rats immunized by other routes. In the fifth experiment, the chemotactic response of neutrophils to casein was shown to be significantly depressed by the addition of bronchial washings obtained from immunized rats (P < 0.01). It is proposed that specific antibody in bronchial washings does not have a direct role in opsonizing bacteria for killing or phagocytosis, but instead has an anti-inflammatory effect. Non-specific effectors such as neutrophils driven by specific immune cells are a likely means of clearance of bacteria following intestinal immunization and acute challenge.

Rats immunized by intra-Peyer's patch (IPP) injection with non-typable Haemophilus influenzae (NTHI) have been shown to clear this organism from the respiratory tract faster than non-immunized rats. We therefore performed a series of experiments in order to determine the mechanism of action of the enhanced pulmonary clearance. The experiments show that homing of intestinal T cells to the respiratory tract is an important component in the observed immunity, while specific antibody adsorbed to bacteria does not influence pulmonary bacterial clearance rate. Mucosally derived lymphocytes were collected from the thoracic duct of rats primed by IPP inoculation with NTHI, and intravenously transfused to recipient rats. These rats were shown to clear bacteria from bronchial spaces faster than non-transfused rats, or rats transfused with non-immune lymphocytes. Lymphocytes collected from the spleens of immunized rats were also capable of conferring the ability to accelerate pulmonary clearance. When thoracic duct lymphocytes (TDL) purified for T lymphocytes were transferred to recipients, the NTHI clearance rate was accelerated. In experiments to evaluate the activity of specific antibody, it was demonstrated that NTHI opsonized with antibody from bronchial washings of immunized rats was not cleared from the respiratory tract of naive rats faster than non-opsonized controls. These data indicate that immune clearance of NTHI from the respiratory tract following gut immunization is dependent upon antigen-primed lymphocytes, that primed T cells are capable of conferring this protection, and that a primary role for specific antibody in the process cannot be established.

An animal model of the clearance of nontypable Haemophilus influenzae has been developed to determine both optimal immunization strategies for controlling colonization of the respiratory tract in patients with damaged airways and mechanisms of action of immune clearance. It was demonstrated that stimulation of gut-associated lymphoid tissue (GALT) (either by direct injection or by ingestion of antigen) followed by local administration of antigen into the bronchus was required to enhance clearance in this model. The primary effect of GALT immunization persisted for at least 6 wk; it was specific and could not be replaced by systemic immunization. Failure to stimulate locally in the bronchus was associated with protracted clearance. No clear correlation between local or systemic antibody and bacterial clearance was demonstrated; however, immunized rats were shown to have faster recruitment of phagocytic cells to the bronchial spaces, and these phagocytes had a higher activity state than cells harvested from nonimmunized animals. It is probable that bacterial clearance is accelerated in immunized animals due in part to factors mediating a change in the behavior of luminal phagocytes.