A major roadblock in the development of novel vaccines is the formulation and delivery of the antigen. Liposomes composed of a dimethyldioctadecylammonium (DDA) backbone and the adjuvant trehalose-6-6-dibehenate (TDB, termed "cationic adjuvant formulation (CAF01)", promote immunogenicity and protective efficacy of vaccines, most notably against infection with Mycobacterium tuberculosis. Specifically, the multicomponent antigen H56 delivered by CAF01 protects against tuberculosis in mice. Here we investigated whether the inclusion of immune-modulatory adjuvants into CAF01 modulates the immunogenicity of H56/CAF01 in vitro and in vivo. Based on our recent findings we selected the active sequence of the mycobacterial 19 kDa lipoprotein, Pam3Cys, which interacts with Toll like receptor 2 to induce an antimicrobial pathway. H56/CAF01-Pam3Cys liposomes were characterized for Pam3Cys incorporation, size, toxicity and activation of primary human macrophages. Macrophages efficiently take up H56/CAF01-Pam3Cys and trigger the release of significantly higher levels of TNF, IL-12 and IL-10 than H56/CAF01 alone. To evaluate the immunogenicity in vivo, we immunized mice with H56/CAF01-Pam3Cys and measured the release of IFN-γ and IL-17A by lymph node cells and spleen cells. While the antigen-specific production of IFN-γ was reduced by inclusion of Pam3Cys into H56/CAF01, the levels of IL-17A remained unchanged. In agreement with this finding, the concentration of the IFN-γ-associated IgG2a antibodies in the serum was lower than in H56/CAF01 immunized animals. These results provide proof of concept that Toll like-receptor agonist can be included into liposomes to modulate immune responses. The discordant results between the in vitro studies with human macrophages and in vivo studies in mice highlight the relevance and complexity of comparing immune responses in different species.
Keywords: Liposomes; Toll like-receptor agonist; Tuberculosis; Vaccination.