Figure 8.

Replication and pathogenicity of rPR8 and rPR8-NP^Y313F viruses in vitro and in vivo. (A) Six-week-old female BALB/c mice were intranasally infected with 10-fold serial dilutions containing 10¹ to 10⁶ EID₅₀ of rPR8 or rPR8-NP^Y313F virus. Changes in body weight were monitored for 14 days after the viral challenge. (B) BALB/c mice were intranasally infected with 10-fold serial dilutions containing 10¹ to 10⁶ EID₅₀ of rPR8 or rPR8-NP^Y313F virus. The MLD₅₀ was calculated by using the method of Reed and Muench. (C) Evaluation of histopathological changes in the lungs of infected and uninfected mice by means of HE staining on day 3 or 5 post-inoculation. Scale bar: 200 μm. (D) BALB/c mice intranasally infected with rPR8 (10⁶ EID₅₀) or rPR8-NP^Y313F (10⁶ EID₅₀) were euthanized on day 3 post-inoculation, and the nasal turbinates and lungs were collected for viral titration in chicken eggs. (E) BALB/c mice intranasally infected with rPR8-NP^Y313F (10⁶ EID₅₀) were euthanized on days 1, 3, and 5 post-inoculation, and the lung tissue was collected for viral RNA isolation and sequencing. Each bar represents the amino acid at position 313 of the NP protein from an individual animal. Results are representative of three independent experiments.