Objective To compare the pathogenicity of two subtypes of influenza A virus A/ Shanghai/37T/2009(H1N1) (pdm09) and A/ PuertoRico/8/34 (H1N1) (PR8) and the immunogenicity of those two subtypes of influenza Avirus to induce antigen specific CD8⁺ T cells in C57BL/6 mice. Methods ①The wild type C57BL/6 mice were infectedwith 50% tissue infection dose (TCID₅₀) of PR8 and pdm09 respectively. The situations of the infected mice were observeddaily (14 days), and the death rate was recorded. The mice were sacrificed at 1 and 3 days (days post infection, d.p.i),the lungs were collected to weigh and the lung indexes were calculated, and the left lobes of lung were fixed by 4%paraformaldehyde and stained with H&E to observe the pathological changes of the lung after infection. The viral load inlung tissue was detected by real-time fluorescence quantitative PCR (qRT-PCR). ② The mice were intranasally infectedwith 0. 25 times lethal dose (lethal dose 50%, LD50) of PR8 and pdm09, respectively. The infected mice were weighedevery day and monitored continuously for 14 days. The splenocytes were isolated on the 8 d.p.i. The percentages of influenzaviral antigen specific CD8⁺ T cells, the expression levels of IFN-γ, TNF-α, IL-2, granzyme B and immunosuppressivecheckpoint molecules PD-1, LAG-3, Tim-3 and CTLA-4 were detected by fluorescence-activated cell sorting (FACS).Results ①After the same TCID₅₀ influenza virus infections, the mice in both groups were morbid. All the mice in thePR8 group died on the 5th day after infection, and the mortality rate in the PR8 group was significantly higher than thatin the pdm09 group on the 1st and 3rd day ( P <0. 01), and the lung index and viral load on the 1st and 3rd day in thePR8 group were significantly higher than those in the pdm09 group. The result of H&E staining showed that thepathological injury of lung tissue was mild and the inflammatory infiltration was less in the pdm09 group. ② After thesame LD₅₀ influenza virus infection, the weight-loss and pathological changes in the lung tissues in the PR8 group weresignificantly more serious than in the pdm09 group. The result of flow cytometry showed that the proportion of virusspecificCD8⁺ T cells in the PR8 group was higher than that in the pdm09 group, but the productions of IFN-γ, TNF-αand IL-2 by activated CD8⁺ T cells in the PR8 group were significantly lower than that in pdm09 group. Further detectionof exhaustion makers on the activated CD8⁺ T cells revealed that the cells of the PR8 group express moreimmunosuppressive molecules PD-1 and LAG-3 than those cells of the pdm09 group. Conclusions Influenza A virusPR8 can cause more serious damage in mice compared with pdm09, which may be due to the functional exhaustion ofactivated antigen specific CD8⁺ T cells, resultsing in lower protective immunity against influenza viral infection and more severe pathological injury than pdm09 infection.