Characterisation of heterologous prime-boost vaccination strategies : an investigation into the nature and delivery of vaccines and the subsequent generation of immune responses
This thesis describes work undertaken to consider how strategies of vaccination can be manipulated to generate specific types and magnitude of immune response to prevent infection or treat established infection. The vaccination strategies employed were based on the initial delivery of DNA vaccine through intramuscular injection or ballistic delivery using a gene gun, followed by heterologous boosts, based on the same antigen delivered using an alternative route. These boost strategies included a) intramuscular delivery of purified recombinant HBcAg, b) mucosal delivery of HBcAg expressed by an attenuated strain of Salmonella typhimurium, and c) intranasal delivery of the protein accompanied by a mucosal adjuvant. Following each vaccination regime tested a number of specific immune responses, including serum and mucosal antibody production, CD4+ T helper proliferation in spleens and lymph nodes, CD8+ T cell activation and killing were measured. These experiments revealed that the character of the immune responses primed in response to DNA vaccination differed according to route of immunisation, with intramuscular vaccination inducing a rapid CTL response. CD4+ T cells generated appeared to be of the Th1 phenotype and showed the strongest localisation in the spleen. In contrast, following vaccination with the gene gun, CD4+ cells of a Th2 phenotype were generated with responses being found to be stronger in the local draining lymph nodes that the spleen. Combining DNA delivered using the intramuscular route with recombinant purified protein delivered by the same route was effective at boosting systemic antibody titres, as was boosting using attenuated Salmonella expressing the same antigen when delivered to the mucosal surface of the gut. In contrast, boosting of DNA primed animals with purified protein, even in the presence of an appropriate mucosal adjuvant, was not successful at increasing titres of systemic antibodies. Interestingly, specific mucosal immune responses were absent when either of the heterologous vaccination strategies used a mucosal boosting approach.