2.3. Virus. The influenza A/FM/1/47(H1N1) virus was pro- vided by the College of Veterinary Medicine of South China Agricultural University (Guangzhou, China). The virus strain was propagated in specific pathogen-free fertilized eggs and adapted for lethality in mice after three passages in the animal. Virus-containing allantoic fluid was harvested and stored in aliquots at −80∘C. The LD50 was determined in mice after serial dilution of the stock. Amounts equal to twice the LD50 value were used for viral challenge in all of the exper- iments. Infection was established by intranasal inoculation
in mice anesthetized by ethyl ether. The influenza-related pathogenic operation was performed in the Animal Biosafety Level 3 Laboratory in South China Agriculture University.