Infiltration and activation of neutrophils in lung tissues during Chlamydia muridarum respiratory tract infection
Objective To investigate the possible mechanism of aggregation and activation of neu-trophils（polymorphonuclear neutrophils,PMN）in mice with chlamydial pneumonitis. Methods C57BL/ 6 mice were inoculated intranasally with 3×10^3 inclusionforming units（IFU）of Chlamydia muridarum（Cm） to induce the murine model of chlamydial pneumonitis. Samples of lung tissues collected at different time points after infection were stained by hematoxylin and eosin for histopathological assessment of inflammation. The levels of myeloperoxidase（MPO）were detected for the evaluation of PMN aggregation. The mononu-clear cells were isolated from lung tissues. The inflammatory cells were counted with Giemsaˊs staining. CD11b^＋Gr1^＋ cell population and CD11b expression in lung mononuclear cells were analyzed by flow cytome-try. The expression of chemokines（MIP-2,LIX,KC and MCP-1）in lung tissues at mRNA level was meas-ured by RT-PCR. Results Chlamydial pneumonitis was induced in mice by intranasal inoculation of 3×10^3 IFU of Cm. Compared with the mice from control group,large amounts of inflammatory cells including PMN, monocytes and lymphocytes were induced in lung tissues of mice with Cm infection. PMN responded earlier than monocytes to the infection. The levels of MPO were significantly increased in mice with Cm infection and reached the highest level on the 7th day after infection. A decline in MPO levels was observed on the 14th day but the levels were still higher than those on day 0. The percentages and total numbers of CD11b^＋Gr1^＋ cells were significantly increased after Cm infection. Moreover,an increased expression of PMN CD11b was also detected by flow cytometry. The expression of chemokines（MIP-2,LIX,KC and MCP-1）was in-creased in lung tissues of mice after Cm infection. The results of the study indicated that Cm infection induced the expression of PMN chemoattractants,resulting in the recruitment of PMN. Conclusion The infiltration and activation of PMN in lung tissues of mice were induced by Cm infection through increasing the expression of chemokines. PMN played an important role in immune responses against Cm infection. 
 
Key words: 
Chlamydial pneumonitis; PMN; Chemokines