No neutralizing activity was detected in the sera of rPIV5-RSV-G-immunized mice ( Fig. 4). Four days post-challenge, RSV A2 titers were measured in the lungs to assess the efficacy of the recombinant vaccine viruses in reducing viral burden. Mice vaccinated with either rPIV5-RSV-F or rPIV5-RSV-G had no detectable challenge virus in the lungs. In the RSV A2-immunized group, one mouse had a viral titer of 90 PFU/lung, while all other mice in the group had no detectable virus. Mice with PBS had an average viral titer of

4.5 × 103 PFU/lung (Fig. 5). Therefore, immunization with the vaccine candidates induced potent immunity against RSV A2 challenge. Lung histology was performed to determine if immunization with the recombinant vaccine viruses affected RSV-induced lung pathology. At low magnification, tissue from mice find more vaccinated with RSV A2 or the rPIV5 viruses showed similar levels of inflammatory

infiltrates 4 days post-challenge. Lung LY2157299 tissue from the mock-vaccinated mice was the least inflamed (Fig. 6A–D), suggesting that vaccinated animals had likely generated immune responses to RSV challenge. At high magnification, the inflammation in the mice vaccinated with RSV A2 or the recombinant vaccine viruses was characterized most prominently by perivascular cuffing (Fig. 7A and B). The leukocytes surrounding the pulmonary blood vessels consisted of mostly lymphocytes and macrophages, with few neutrophils and eosinophils. Mild-to-moderate interstitial pneumonia (Fig. 7A and C) and little-to-no bronchiolitis (Fig. 7A and D) was observed in all groups.

Tissue sections were also scored for alveolitis, pleuritis, and vasculitis (Fig. 7E–G). There most were no significant differences in the histopathology scores of mice vaccinated with the recombinant vaccine viruses relative to the RSV A2-vaccinated controls. The most advanced area of investigation for RSV vaccine candidates is live attenuated viruses. These viruses have several benefits: (1) enhanced RSV disease has not been observed either after natural infection or vaccination with live attenuated viruses [32], [33] and [34]; (2) live attenuated RSV vaccines induce balanced immune responses that more closely match natural immunity compared with subunit or inactivated vaccines [35] and [36]; (3) intranasal vaccination with live attenuated viruses should induce better local immunity compared with intramuscular injection of subunit vaccines. Live attenuated RSV vaccines have been in development for several decades, using a combination of cold passage (cp) and chemical mutagenesis to induce temperature sensitivity (ts). A number of cpts RSV vaccine candidates have been tested clinically. The cpts 248/404 candidate was sufficiently attenuated in adults and sero-negative children and tested in 1 to 3-month-old infants. However, cpts 248/404 caused nasal congestion in these infants, an unacceptable adverse effect [32].