Abstract： Objective:To investigate the role of caspase recruitment domain protein 9 (CARD9) in airway injury and inflammation of steroid resistant asthma in C57BL/6 mice.Methods:C57BL/6 mice were divided into A group (control group), B group (model group) and C group (dexamethasone treatment group), with 6 mouse in each group using random number table. The mouse asthma model was established in B and C group by subcutaneous injection of ovalbumin (OVA)/complete Freund adjuvant (CFA) in the abdomen and OVA aerosol challenge, the pathological change and cell count in broncho alveolar lavage fluid (BALF) were detected in order to confirm the model as steroid resistant asthma, and the lung tissue inflammatory infiltration was scored. Western blot was used to detect the changes of CARD9 protein between the group A and B; then wild-type and CARD9 knockout mice were divided into D group (wild-type control group), E group (wild-type model group), F group (CARD9 knockout control group) and G group (CARD9 knockout model group), the following indicators were observed and compared after establishing steroid resistant asthma model separately: HE staining was used to observe the pathological changes of lung tissue, ELISA was used to detect the protein levels of interleukin-4 (IL-4), interleukin-5 (IL-5) and interleukin-17(IL-17) in BALF, and RT-PCR was used to detect the mRNA levels of CXC motif chemokine ligand-10 (CXCL-10) and IL-17 in lung.Results:The inflammatory score (3.33±0.82 vs 0.67±0.52) and BALF total cell count [(10.13±4.83) ×10 5/ml vs (3.76±0.84) ×10 5/ml] in B group were higher than those in the A group with statistical significance ( P<0.05). There was no significant difference between group C and group B in inflammatory infiltration score (2.83±0.75 vs 3.33±0.82) and BALF total cell count [(9.80±3.19) ×10 5/ml vs (10.13±4.83) ×10 5/ml] ( P>0.05). Moreover the protein level of CARD9 was increased in the B group than A group (0.245±0.090 vs 0.047±0.014, P=0.004). Compared to E group and F group, more obviously inflammatory cells, neutrophils, eosinophils infiltration and tissue injury were observed in G group ( P<0.05), so did the expression of IL-4 ( P<0.05), IL-5 and IL-17. Meanwhile the mRNA expression levels of IL-17 and CXCL-10 also increased in lung tissue ( P<0.05) of G group. Conclusion:CARD9 gene deletion may aggravate the steroid resistant of asthma by increasing neutrophil chemokines, such as IL-17 and CXCL-10, therefore increasing infiltration of neutrophils in C57BL/6 mice asthma model.