The dynamic change of cytokines in lung tissues of mice in the early phase of Streptococcus pneumoniae infection

The study was designed to establish nonfatal Streptococcus pneumoniae mouse model and investigate the role of the cytokines in clearing Streptococcus pneumoniae within 72 hours after infection.Adult SPF Balb/c mice(female,6-8 week,15-18 g) were infected with 50 μl Streptococcus pneumoniae D39 by nose dropping at the concentration of 3×106 cfu/ml.Then BALF of experimental mice were collected for bacterial counting and cells counting;the lung tissues were collected for bacterial counting,histopathological examination and cytokine detection.We found the bacterial population of the BALF dropped gradually at 2 h after inoculation,then dropped to normal level at 72 h after inoculation.The bacterial population of the lung homogenate peaked at 6 h after inoculation,and dropped obviously from 6 h to 24 h,and mostly cleared in 72 h after inoculation.The percentage of lung neutrophils increased obviously at 2 h after inoculation,and peaked at 12 h,then dropped to normal level at 48 h.The percentage of lymphocytes in lung homogenate increased obviously at 2 h after inoculation,and peaked at 6 h,then dropped to normal level at 24 h.The inflammation cell infiltration in lung tissue was found at 6 h after inoculation,peaked at 48 h,and then lung tissue began to recover.The levels of pro-inflammation cytokines in lung homogenate such as IFN-γ,TNF-α,IL-6,IL-1 and IL-17 were significantly higher(P 0.05) at 2 h after inoculation,then the levels of IFN-γ,TNF-α,IL-6 and IL-1 dropped gradually to normal after 6 h,while the level of IL-17 kept high level in the first 12 h after inoculation(P 0.05) then dropped to normal level.In conclusion,the changes of the cytokines in lung tissues of early phase of infection may play an important role not only in the innate immune response but also in initiating adaptive immune response.The dynamic balance of cytokines could promote the recruitment of neutrophils and lymphocytes to clear the bacteria,at the same time avoid lung tissue damage caused by the excessive inflammatory response.