Efficacy and safety of an intranasal virosomal respiratory syncytial virus vaccine adjuvanted with monophosphoryl lipid A in mice and cotton rats.

Respiratory syncytial virus infection remains a serious health problem, not only in infants but also in immunocompromised adults and the elderly. An effective and safe vaccine is not available due to several obstacles: non-replicating RSV vaccines may prime for excess Th2-type responses and enhanced respiratory disease (ERD) upon natural RSV infection of vaccine recipients. We previously found that inclusion of the Toll-like receptor 4 (TLR4) ligand monophosphoryl lipid A (MPLA) in reconstituted RSV membranes (virosomes) potentiates vaccine-induced immunity and skews immune responses toward a Th1-phenotype, without priming for ERD. As mucosal immunization is an attractive approach for induction of RSV-specific systemic and mucosal antibody responses and TLR ligands could potentiate such responses, we explored the efficacy and safety of RSV-MPLA virosomes administered intranasally (IN) to mice and cotton rats. In mice, we found that incorporation of MPLA in IN-administered RSV virosomes increased both systemic IgG and local secretory-IgA (S-IgA) antibody levels and resulted in significantly reduced lung viral titers upon live virus challenge. Also, RSV MPLA virosomes induced more Th1-skewed responses compared to responses induced by FI-RSV. Antibody responses and Th1/Th2-cytokine responses induced by RSV-MPLA virosomes were comparable to those induced by live RSV infection. By comparison, formalin-inactivated RSV (FI-RSV) induced serum IgG that inhibited viral shedding upon challenge, but also induced Th2-skewed responses. In cotton rats, similar effects of incorporation of MPLA in virosomes were observed with respect to induction of systemic antibodies and inhibition of lung viral shedding upon challenge, but mucosal sS-IgA responses were only moderately enhanced. Importantly, IN immunization with RSV-MPLA virosomes, like live virus infection, did not lead to any signs of ERD upon live virus challenge of vaccinated animals, whereas IM immunization with FI-RSV did induce severe lung immunopathology under otherwise comparable conditions. Taken together, these data show that mucosally administered RSV-MPLA virosomes hold promise for a safe and effective vaccine against RSV.