Induction of tolerance versus immunity following pulmonary vaccination

When targeting mucosal pathogens, it is generally accepted that vaccination at mucosal sites will generate a more efficient mucosal immune response. As influenza is an infection of the airways, a vaccine that targets the actual site of infection should result in a more efficient immune response as compared to the conventionally delivered vaccines. The lungs are continuously exposed to environmental antigens and allergens, and the induction of immune responses against these molecules would result in continuous immune stimulation, which would eventually harm the host. As a result, immune responses in the lungs need to be tightly regulated to prevent a hypersensitive response leading to inflammation. Exposure of antigens to the lung often leads to induction of tolerance rather than immunisation, therefore, the lung may not be the most obvious site for induction of mucosal immunity. For a pulmonary vaccine to work, this tolerance first needs to be understood and overcome so that a protective immune response can be generated. In order to develop a pulmonary vaccine, the challenge of inducing immunity rather than tolerance must be overcome. The specific aims of this thesis were to i) determine if mucosal tolerance is induced in the lung following exposure to influenza antigen without an adjuvant, and if so what this mechanism is, and ii) determine the mechanism responsible for avoiding this tolerance leading to an immune response after pulmonary vaccination with an influenza ISCOMATRIX(TM) vaccine. Pulmonary vaccines were delivered to sheep, and antibody titres in both the lung and the blood were determined through ELISA. Tolerance-associated markers and cytokines were also measured after vaccination through the use of qPCR. It was found that if sheep were vaccinated with an initial dose of influenza antigen followed by an influenza ISCOMATRIX(TM) vaccine, the antibody response generated was significantly reduced compared to animals that received an influenza ISCOMATRIX(TM) vaccine only. Results suggest that this initial exposure to influenza antigen may cause a tolerizing effect in the lung. In contrast, the expression of the regulatory T-cell marker FOXP3 and regulatory cytokines (associated with the induction of tolerance) were significantly reduced in vaccinated animals in the presence of adjuvant, indicating that regulatory T-cells may need to be suppressed in order to generate an efficient immune response. This decrease in a regulatory response should indicate the production of either cell-mediated or a humoral immune response, however, it was found that neither a Th1, Th2, or Th17 response was generated following pulmonary vaccination with an influenza ISCOMATRIX(TM) vaccine. In the long-term, it is hoped that these improvements might contribute towards making lung-delivery of influenza vaccines a more feasible option in humans