Evaluation of an Intranasal Virosomal Vaccine against Respiratory Syncytial Virus in Mice: Effect of TLR2 and NOD2 Ligands on Induction of Systemic and Mucosal Immune Responses

INTRODUCTION: RSV infection remains a serious threat to newborns and the elderly. Currently, there is no vaccine available to prevent RSV infection. A mucosal RSV vaccine would be attractive as it could induce mucosal as well as systemic antibodies, capable of protecting both the upper and lower respiratory tract. Previously, we reported on a virosomal RSV vaccine for intramuscular injection with intrinsic adjuvant properties mediated by an incorporated lipophilic Toll-like receptor 2 (TLR2) ligand. However, it has not been investigated whether this virosomal RSV vaccine candidate would be suitable for use in mucosal immunization strategies and if additional incorporation of other innate receptor ligands, like NOD2-ligand, could further enhance the immunogenicity and protective efficacy of the vaccine. OBJECTIVE: To explore if intranasal (IN) immunization with a virosomal RSV vaccine, supplemented with TLR2 and/or NOD2-ligands, is an effective strategy to induce RSV-specific immunity. METHODS: We produced RSV-virosomes carrying TLR2 (Pam(3)CSK(4)) and/or NOD2 (L18-MDP) ligands. We tested the immunopotentiating properties of these virosomes in vitro, using TLR2- and/or NOD2-ligand-responsive murine and human cell lines, and in vivo by assessing induction of protective antibody and cellular responses upon IN immunization of BALB/c mice. RESULTS: Incorporation of Pam(3)CSK(4) and/or L18-MDP potentiates the capacity of virosomes to activate (antigen-presenting) cells in vitro, as demonstrated by NF-κB induction. In vivo, incorporation of Pam(3)CSK(4) in virosomes boosted serum IgG antibody responses and mucosal antibody responses after IN immunization. While L18-MDP alone was ineffective, incorporation of L18-MDP in Pam(3)CSK(4)-carrying virosomes further boosted mucosal antibody responses. Finally, IN immunization with adjuvanted virosomes, particularly Pam(3)CSK(4)/L18-MDP-adjuvanted-virosomes, protected mice against infection with RSV, without priming for enhanced disease. CONCLUSION: Mucosal immunization with RSV-virosomes, supplemented with incorporated TLR2- and/or NOD2-ligands, represents a promising approach to induce effective and safe RSV-specific immunity.