Immunization with 30 μg adjuvanted RSV F nanoparticles elicited significantly higher serum levels of PCA (884 μg/ml) than animals that received 15 mg/kg (human MG-132 supplier dose) of palivizumab (86 μg/ml). PCA was below the LOD of the assay (<20 μg/ml) in cotton rats immunized with FI-RSV, and naïve control groups, and slightly above LOD in the RSV A intranasal immunization group (Fig. 1B). Sera from all groups, with the exception of

FI-RSV and placebo recipients, had virus neutralizing antibodies (Fig. 1C). Adjuvanted RSV F elicited higher neutralization titers (GMT = 697) than natural infection (GMT = 95) or palivizumab passively immunized cotton rats (GMT = 320) (Fig. 1C). The neutralizing titer differences observed between cotton rats that received adjuvanted RSV F and virus infected cotton rats were statistically significant (p < 0.01) following the same trend observed from analysis of PCA and anti-RSV F ELISA responses. The in vivo efficacy of RSV F nanoparticle vaccine was evaluated by measuring inhibition of viral

replication in the lungs and nasal passages of immunized cotton rats challenged with RSV. Complete inhibition of virus replication was observed in the lungs of cotton rats immunized with live RSV, RSV F nanoparticles administered with and without adjuvant, as well as palivizumab Ribociclib mouse given passively ( Fig. 2A). FI-RSV reduced lung viral load (pfu/g tissue; GMT = 2357) when compared to naïve challenged cotton rats (pfu/g tissue; GMT = 194,237) but failed to confer full protection. When viral replication was evaluated in the nasal compartment, only the RSV F vaccine with adjuvant and RSV infection groups were completely protected ( Fig. 2B). Cotton rats that received unadjuvanted RSV F and palivizumab had reduced viral load compared to the naïve animal group but with readily

measurable virus titers in nasal tissue following challenge ( Fig. 2B). When Lot 100 FI-RSV vaccine was used in a clinical trial in the late 1960s, vaccinated children developed enhanced respiratory disease (ERD) upon reinfection [33]. Similarly, ERD can be reproduced in the cotton rat model with the same vaccine, known as Lot 100 FI-RSV vaccine [30] and [31]. In the current until study, Lot 100 FI-RSV induced prominent alveolitis and perivasculitis in the lungs of RSV challenged animals, consistent with ERD. Conversely, significant lung histopathological changes of this magnitude were not observed in cotton rats immunized with the RSV F nanoparticle vaccine administered with or without adjuvant and were similar to the minimal changes seen in placebo and palivizumab animals (Fig. 3A–C). The RSV F vaccine was derived from the RSV A long sequence. A dose ranging immunization with the RSV F vaccine was undertaken to compare the protective efficacy of the vaccine against a non-homologous challenge (RSV B) with palivizumab, known to be protective against both RSV A and B [34]. Cotton rats were immunized with 0.003, 0.03, 0.3 or 3.