Protection against non-typable Haemophilus influenzae following sensitization of gut associated lymphoid tissue: role of specific antibody and phagocytes.

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.