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Objective To investigate the pathogenesis and immunogenicity of H9N2 influenza virus A/Guangzhou/333/99 (a reassortant of (G1 and G9 viruses isolated from a female patient in 1999) in a mouse model of infection.Methods Mice were infected with increasing virus titers.Viral load in the lungs and trachea was determined by EID50 assay.Pulmonary histopathology was assessed by hematoxylin-eosin staining.Anti-HI antibody titers and T-cell responses to viral HA were determined by ELISPOT and confirmed by flow cytometry.Results Mice presented a mild syndrome after intranasal infection with A/Guangzhou/333/99 (H9N2) influenza virus.Virus was detected in the trachea and lungs of mice harvested on days 3,6,and 9 post-infection.A T-cell response to viral HA was detected on day 6 and H9 HA-specific CD4+ T-cells predominated.Seroconversion was detected after 14 days and antibody persisted for at least 28 weeks.Conclusion Our results suggest that H9N2 (A/Guangzhou/333/99) can replicate in the murine respiratory tract without prior adaptation,and both humoral and cell-mediated immunity play an important role in the immune response.