An investigation of the innate inflammatory properties of immune stimulating complexes (ISCOMs) and their importance for the generation of adaptive immunity

Immune stimulating complexes (ISCOMS) are small cage like particles which form spontaneously upon agitation of phospholipid, cholesterol and the saponin adjuvant Quil A. It is possible to incorporate protein antigen into the ISCOMS structure and ISCOMS have been used to stimulate antigen-specific immune responses by both parenteral and mucosal routes. However, the mechanisms that underlie these adjuvant effects have not been fully elucidated. This project was undertaken in order to expand the knowledge of the adjuvant properties of ISCOMS and to identify the influence that these had on the ability of ISCOMS to drive an antigen-specific immune response. My initial experiments concentrated on performing a detailed study of the innate effects of ISCOMS in vivo, when given initially as an intraperitoneal injection and later orally. The results presented demonstrate that ISCOMS have an ability to stimulate an innate inflammatory cascade, comprised of the recruitment and activation of an inflammatory infiltrate, secretion of inflammatory cytokines and mediators. Secretion of these factors was dependent on the presence of adherent cells indicating the possible role of macrophages in their production. A similar pattern of cellular recruitment and activation was measured in gut associated lymphoid tissues after oral administration of ISCOMS, but this was much less dramatic than had been observed earlier. I then undertook an examination of the influence that these inflammatory mediators had on the generation of antigen-specific immunity as determined by antigen-specific DTH, proliferation, cytokine production, CTL activity serum IgG isotype production and intestinal IgA production, using gene-targeted knockout mice including IL-12KO, IL-6KO, IL-18KO, IL-4KO, iNOSKO and gammaIFNRKO. My experiments revealed that most of the components of the inflammatory response were not essential for the development of antigen-specific immunity to ISCOMS, but it did reveal that IL-12 played a particularly important role in the immunogenicity of ISCOMS, as immune responses were severely defective in these mice after either parenteral or oral immunisation with OVA ISCOMS. I then went on to determine if the generation of normal antigen-specific immune response, in the absence of inflammatory cytokines, was due to the maintenance of the inflammatory cascade induced by ISCOMS. I therefore studied the cellular recruitment and inflammatory mediator release from ISCOMS injected gene targeted knockout mice. Despite most knockout animals retaining an intact innate cascade, the IL-12KO mice with decreased antigen-specific immune responses to ISCOMS also had an ablated innate response. However, unusually the gammaIFNRKO mice which had normal antigen-specific responses to ISCOMS also lacked an innate inflammatory cascade. This apparent dichotomy indicated that the role of the ISCOMS induced innate immune response for the generation of antigen- specific immunity is as complex as it is important and must be better understood if efficacious and potent vaccines are to be developed.