The Use of a Transgenic Mouse Model to Investigate Adjuvant Activity of Mycobacterial Cell Wall Products

Abstract

Vaccines aimed at intracellular pathogens require cell mediated immunity to be stimulated, with the production of Type 1 cytokines such as interleukin-12 (IL-12) by dendritic cells and interferon-γ (IFN-γ) release from T helper cells. Adjuvants are compounds which enhance the immune response to antigens and the use of adjuvants may help to direct the immune response towards the desired profile. Because modern vaccines are required to be safe and with defined components, adjuvants must be developed towards this goal. A number of adjuvants, such as Complete Freund's adjuvant (CFA), are Th1 type adjuvants. However, the adverse effects of these crude products preclude their use in humans. In this study, purified native mycobacterial products, specifically the lipoglycans lipoarabinomannan (LAM), phosphatidylinositol mannosides (PIMs) and synthetic derivatives of PIMs, were investigated for their Th1 type adjuvant properties. A transgenic mouse system was used for the initial screening of the adjuvant properties of lipoglycans because ovalbumin transgenic-II (OT-II) mice have increased numbers of ovalbumin specific MHC restricted αβ T cell receptors. This increase amplifies immune response development and enables detection of immune response changes at an earlier time point compared to non-transgenic mice. Following vaccination using ovalbumin as the model antigen and lipoglycan adjuvants, it was shown that both native and synthetically produced PIMs induced a strong interferon-γ (IFN-γ) response, with a minimal induction of the Th2 cytokines interleukin-4 (IL-4) and interleukin-5 (IL-5). When incubated with dendritic cells in vitro, some of the PIMs were shown to strongly induce the release of interleukin-12 (IL-12). This cytokine is important in the initiation of a Th1 response. The PIMs were shown to be active as adjuvants using the sub-cutaneous immunisation route and also the oral and intra-nasal mucosal routes. Negligible site lesions were observed after sub-cutaneous immunisation when PIMs were used as adjuvants. These results show that it is possible to utilise the adjuvant properties of defined products from mycobacteria without the toxicity that has previously been intrinsic with the use of crude extracts. Overall, it was found that while several PIM products gave an enhanced immune response, it was the synthetically derived PIM2 molecule that gave the most reproducible results. This study indicates that this product should be investigated further as an adjuvant for vaccines against diseases requiring a Th1 response using a variety of vaccination routes.